Port MUSIC tests from SLI to Python

[JanVogelsang]

This PR ports the cont_proxy MUSIC test from SLI to Python.

@heplesser and I also discovered a bug in MUSIC #3679 related to the cont_proxy test case. However, this PR is already valid even without the bug being fixed, as this specific test setup already works.

[heplesser]

I think we need to move sli2py_music out of the pytests directory, or we need to ignore also sli2py_music here:

nest-simulator/testsuite/do_tests.sh

Line 536 in 84da3b5

--ignore="${PYNEST_TEST_DIR}/mpi" --ignore="${PYNEST_TEST_DIR}/sli2py_mpi" "${PYNEST_TEST_DIR}" 2>&1 | tee -a "${TEST_LOGFILE}"

Otherwise, Pytest test collection finds the test_....py files and tries to run them as normal test scripts.

[JanVogelsang]

We now decided to move the music-tests directory outside of the pytests one as the music tests do not use pytest.

[JanVogelsang]

@heplesser This PR now contains all MUSIC tests, as discussed yesterday.

[heplesser]

@JanVogelsang The music tests failed. From the downloaded artefacts it looks like the first one had too few MPI slots:

Running test 'test_cont_proxy.music' with 5 processes... 
   > --------------------------------------------------------------------------
   > There are not enough slots available in the system to satisfy the 5
   > slots that were requested by the application:

and the second did not find the right files:

Running test 'test_event_proxies_issue-696.music' with 2 processes... 
   > Error during launching of binary ./test_event_proxies_issue-696_sender.py
   > MUSIC: No such file or directory
   > --------------------------------------------------------------------------
   > Primary job  terminated normally, but 1 process returned
   > a non-zero exit code. Per user-direction, the job has been aborted.
   > --------------------------------------------------------------------------
   > Error during launching of binary ./test_event_proxies_issue-696_receiver.py
   > MUSIC: No such file or directory

The first problem needs an --oversubscribe. In do_tests.sh we have quite a bit of a setup to configure MPI properly, look for the MPI_LAUNCHER shell variable and its siblings.