build: Resolve failing tests on macOS

[jjerphan]

Reference Issues/PRs

Remaining tests to fix:

• test_parallel_reads
• test_parallel_reads_arctic
• test_force_compact_symbol_list_lock_held_past_ttl

test_compat_write_old_read_new and test_write_new_read_old are the only remaining skipped tests.

What does this implement or fix?

Any other comments?

Checklist

Checklist for code changes...

• Have you updated the relevant docstrings, documentation and copyright notice?
• Is this contribution tested against all ArcticDB's features?
• Do all exceptions introduced raise appropriate error messages?
• Are API changes highlighted in the PR description?
• Is the PR labelled as enhancement or bug so it appears in autogenerated release notes?

[jjerphan]

The previously failing test completes successfully with one thread:

test_num_storage_operations.py::test_read_as_of_version[MotoNfsBackedS3StorageFixtureFactory-0] PASSED [ 88%]
test_num_storage_operations.py::test_read_as_of_version[MotoNfsBackedS3StorageFixtureFactory-1] PASSED [ 88%]
test_num_storage_operations.py::test_read_as_of_version[MotoNfsBackedS3StorageFixtureFactory-2] PASSED [ 88%]
test_num_storage_operations.py::test_read_as_of_version[MotoNfsBackedS3StorageFixtureFactory-3] PASSED [ 88%]
test_num_storage_operations.py::test_read_as_of_version[MotoS3StorageFixtureFactory-0] PASSED [ 88%]
test_num_storage_operations.py::test_read_as_of_version[MotoS3StorageFixtureFactory-1] PASSED [ 88%]
test_num_storage_operations.py::test_read_as_of_version[MotoS3StorageFixtureFactory-2] PASSED [ 88%]
test_num_storage_operations.py::test_read_as_of_version[MotoS3StorageFixtureFactory-3] PASSED [ 88%]

Let's try to use fork over spawn for multiprocessing on macOS as suggested by @phoebusm.

Edit: it does not work, so the problem needs to be inspected further, for now the test is skipped for those builds.