Problem invoking the test suite on macOS

Hi there,

within #342, I found that I haven't been able to invoke the test suite right away. In order to find out about more details, I tried again and would like to report the outcome here.

While I believe the process hasn't been able to download CrateDB 4.2.0 on the first try, now I am seeing:
```
$ python -m unittest -vvvvv

[...] some OKs

HTTP: http://127.0.0.1:44209
./var/folders/06/w9pzygdj7vx53n_0l9q_lhph0000gn/T/tmp2k7qhukz/crate-4.2.0/bin/crate: line 129: /var/folders/06/w9pzygdj7vx53n_0l9q_lhph0000gn/T/tmp2k7qhukz/crate-4.2.0/jdk/Contents/Home/bin/java: No such file or directory
/var/folders/06/w9pzygdj7vx53n_0l9q_lhph0000gn/T/tmp2k7qhukz/crate-4.2.0/bin/crate: line 129: exec: /var/folders/06/w9pzygdj7vx53n_0l9q_lhph0000gn/T/tmp2k7qhukz/crate-4.2.0/jdk/Contents/Home/bin/java: cannot execute: No such file or directory

[...] some more OKs

ERROR: setUpModule (tests.test_integration)
----------------------------------------------------------------------
Traceback (most recent call last):
File "/Users/amo/dev/crate/sources/crash/tests/test_integration.py", line 46, in setUpModule
node.start()
File "/Users/amo/dev/crate/sources/crash/.venv/lib/python3.8/site-packages/crate/testing/layer.py", line 316, in start
self._wait_for_start()
File "/Users/amo/dev/crate/sources/crash/.venv/lib/python3.8/site-packages/crate/testing/layer.py", line 373, in _wait_for_start
self._wait_for(validator)
File "/Users/amo/dev/crate/sources/crash/.venv/lib/python3.8/site-packages/crate/testing/layer.py", line 352, in _wait_for
raise SystemError('Failed to start Crate instance in time.')
SystemError: Failed to start Crate instance in time.
```

It is probably just a minor issue. I am working on macOS and invoked that within a dedicated virtualenv after installing the package into it with
```
pip install -U -e ".[test]"
```

With kind regards,
Andreas.