Switch tests to use pytest

Looks like `pytest` is not able to find any units:
```console
+ /usr/bin/pytest -ra
=========================================================================== test session starts ============================================================================
platform linux -- Python 3.8.12, pytest-6.2.5, py-1.10.0, pluggy-0.13.1
rootdir: /home/tkloczko/rpmbuild/BUILD/pwntools-4.7.0
plugins: shutil-1.7.0, virtualenv-1.7.0, mock-3.6.1, cov-2.12.1, anyio-3.3.4, forked-1.3.0, xdist-2.3.0, flaky-3.7.0, tornasync-0.6.0.post2, console-scripts-1.2.0, trio-0.7.0, timeout-2.0.1, asyncio-0.16.0, hypothesis-6.27.0
collected 0 items

========================================================================== no tests ran in 6.29s ===========================================================================
```
As well `setuptools` based test suite i sfailing
```console
+ /usr/bin/python3 setup.py test
Failed to convert README.md through pandoc, proceeding anyway
Traceback (most recent call last):
File "setup.py", line 83, in
long_description = subprocess.check_output(['pandoc', 'README.md', '--to=rst'], universal_newlines=True)
File "/usr/lib64/python3.8/subprocess.py", line 415, in check_output
return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
File "/usr/lib64/python3.8/subprocess.py", line 493, in run
with Popen(*popenargs, **kwargs) as process:
File "/usr/lib64/python3.8/subprocess.py", line 858, in __init__
self._execute_child(args, executable, preexec_fn, close_fds,
File "/usr/lib64/python3.8/subprocess.py", line 1704, in _execute_child
raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: 'pandoc'
running test
WARNING: Testing via this command is deprecated and will be removed in a future version. Users looking for a generic test entry point independent of test runner are encouraged to use tox.
/usr/lib/python3.8/site-packages/pkg_resources/__init__.py:116: PkgResourcesDeprecationWarning: 1.16.0-unknown is an invalid version and will not be supported in a future release
warnings.warn(
/usr/lib/python3.8/site-packages/setuptools/installer.py:27: SetuptoolsDeprecationWarning: setuptools.installer is deprecated. Requirements should be satisfied by a PEP 517 installer.
warnings.warn(
running egg_info
writing pwntools.egg-info/PKG-INFO
writing dependency_links to pwntools.egg-info/dependency_links.txt
writing entry points to pwntools.egg-info/entry_points.txt
writing requirements to pwntools.egg-info/requires.txt
writing top-level names to pwntools.egg-info/top_level.txt
listing git files failed - pretending there aren't any
reading manifest file 'pwntools.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
warning: no files found matching '*.sh'
warning: no files found matching '*.rst' under directory 'pwnlib'
warning: no files found matching '*.md' under directory 'pwnlib'
warning: no files found matching '*.asm' under directory 'pwn'
warning: no files found matching '*.rst' under directory 'pwn'
warning: no files found matching '*.md' under directory 'pwn'
warning: no files found matching '*.txt' under directory 'pwn'
warning: no files found matching '*.sh' under directory 'pwn'
warning: manifest_maker: MANIFEST.in, line 7: 'recursive-exclude' expects ...

adding license file 'LICENSE-pwntools.txt'
writing manifest file 'pwntools.egg-info/SOURCES.txt'
running build_ext
Traceback (most recent call last):
File "setup.py", line 88, in
setup(
File "/usr/lib/python3.8/site-packages/setuptools/__init__.py", line 153, in setup
return distutils.core.setup(**attrs)
File "/usr/lib64/python3.8/distutils/core.py", line 148, in setup
dist.run_commands()
File "/usr/lib64/python3.8/distutils/dist.py", line 966, in run_commands
self.run_command(cmd)
File "/usr/lib64/python3.8/distutils/dist.py", line 985, in run_command
cmd_obj.run()
File "/usr/lib/python3.8/site-packages/setuptools/command/test.py", line 223, in run
self.run_tests()
File "/usr/lib/python3.8/site-packages/setuptools/command/test.py", line 226, in run_tests
test = unittest.main(
File "/usr/lib64/python3.8/unittest/main.py", line 100, in __init__
self.parseArgs(argv)
File "/usr/lib64/python3.8/unittest/main.py", line 124, in parseArgs
self._do_discovery(argv[2:])
File "/usr/lib64/python3.8/unittest/main.py", line 244, in _do_discovery
self.createTests(from_discovery=True, Loader=Loader)
File "/usr/lib64/python3.8/unittest/main.py", line 154, in createTests
self.test = loader.discover(self.start, self.pattern, self.top)
File "/usr/lib64/python3.8/unittest/loader.py", line 349, in discover
tests = list(self._find_tests(start_dir, pattern))
File "/usr/lib64/python3.8/unittest/loader.py", line 405, in _find_tests
tests, should_recurse = self._find_test_path(
File "/usr/lib64/python3.8/unittest/loader.py", line 483, in _find_test_path
tests = self.loadTestsFromModule(package, pattern=pattern)
File "/usr/lib/python3.8/site-packages/setuptools/command/test.py", line 56, in loadTestsFromModule
tests.append(self.loadTestsFromName(submodule))
File "/usr/lib64/python3.8/unittest/loader.py", line 191, in loadTestsFromName
return self.loadTestsFromModule(obj)
File "/usr/lib/python3.8/site-packages/setuptools/command/test.py", line 56, in loadTestsFromModule
tests.append(self.loadTestsFromName(submodule))
File "/usr/lib64/python3.8/unittest/loader.py", line 205, in loadTestsFromName
test = obj()
TypeError: adb() missing 1 required positional argument: 'argv'
```