The 'test' target rebuilds somethig even though the project was already built

Nobody has claimed this yet.

See the log below.

'test' should only test when the build was done previously.

Please add to the README what is the proper command line to test it.

```
===> Testing for py37-zstandard-0.15.1_1
===> py37-zstandard-0.15.1_1 depends on file: /usr/local/bin/python3.7 - found
generating build/zstandard/_cffi.c
(already up-to-date)
not modified: 'build/zstandard/_cffi.c'
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.
running egg_info
writing zstandard.egg-info/PKG-INFO
writing dependency_links to zstandard.egg-info/dependency_links.txt
writing requirements to zstandard.egg-info/requires.txt
writing top-level names to zstandard.egg-info/top_level.txt
reading manifest file 'zstandard.egg-info/SOURCES.txt'
reading manifest template 'MANIFEST.in'
writing manifest file 'zstandard.egg-info/SOURCES.txt'
running build_ext
building 'zstandard.backend_c' extension
cc -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -fno-omit-frame-pointer -isystem /usr/local/include -fPIC -Ic-ext -Izstd -I/usr/local/include/python3.7m -c c-ext/backend_c.c -o build/temp.freebsd-12.2-STABLE-amd64-3.7/c-ext/backend_c.o -DZSTD_SINGLE_FILE -DZSTDLIB_VISIBILITY= -DZDICTLIB_VISIBILITY= -DZSTDERRORLIB_VISIBILITY= -fvisibility=hidden
cc -shared -fstack-protector-strong -O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -fno-omit-frame-pointer -isystem /usr/local/include build/temp.freebsd-12.2-STABLE-amd64-3.7/c-ext/backend_c.o -L/usr/local/lib -lpython3.7m -o /usr/ports/archivers/py-zstandard/work-py37/zstandard-0.15.1/zstandard/backend_c.so
building 'zstandard._cffi' extension
cc -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -fno-omit-frame-pointer -isystem /usr/local/include -fPIC -I/usr/ports/archivers/py-zstandard/work-py37/zstandard-0.15.1/zstd -I/usr/ports/archivers/py-zstandard/work-py37/zstandard-0.15.1/zstd/dictBuilder -I/usr/local/include/python3.7m -c build/zstandard/_cffi.c -o build/temp.freebsd-12.2-STABLE-amd64-3.7/build/zstandard/_cffi.o
cc -Wno-unused-result -Wsign-compare -Wunreachable-code -DNDEBUG -O2 -pipe -fstack-protector-strong -fno-strict-aliasing -O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -fno-omit-frame-pointer -isystem /usr/local/include -fPIC -I/usr/ports/archivers/py-zstandard/work-py37/zstandard-0.15.1/zstd -I/usr/ports/archivers/py-zstandard/work-py37/zstandard-0.15.1/zstd/dictBuilder -I/usr/local/include/python3.7m -c zstd/zstdlib.c -o build/temp.freebsd-12.2-STABLE-amd64-3.7/zstd/zstdlib.o
cc -shared -fstack-protector-strong -O2 -pipe -fno-omit-frame-pointer -fstack-protector-strong -isystem /usr/local/include -fno-strict-aliasing -fno-omit-frame-pointer -isystem /usr/local/include build/temp.freebsd-12.2-STABLE-amd64-3.7/build/zstandard/_cffi.o build/temp.freebsd-12.2-STABLE-amd64-3.7/zstd/zstdlib.o -L/usr/local/lib -lpython3.7m -o /usr/ports/archivers/py-zstandard/work-py37/zstandard-0.15.1/zstandard/_cffi.so
test_arguments (tests.test_buffer_util.TestBufferWithSegments) ... ok
test_invalid_getitem (tests.test_buffer_util.TestBufferWithSegments) ... ok
test_invalid_offset (tests.test_buffer_util.TestBufferWithSegments) ... ok
```

Start with the project's test target and README, then reproduce the logged behavior after a prior build. Determine why the test command rebuilds the extensions, document the proper test command in the README, and verify that testing an already-built project no longer triggers the rebuild.

Written by the indexing model from the issue text.