test(profiling): add a test for multiprocessing

[jd]

We test fork(), but we don't test higher level frameworks.
Most Python frameworks using multiple processes are based on multiprocessing.
Add a simple test that shows we do collect profiles for subprocesses when using
multiprocessing.

[mtoffl01]

looks good to me!

[Kyle-Verhoog]

@jd looks like the path doesn't match up:

>           fileobj = self.myfileobj = builtins.open(filename, mode or 'rb')
E           FileNotFoundError: [Errno 2] No such file or directory: '/tmp/pytest-of-root/pytest-4/test_multiprocessing0/pprof.3299.1'

../.pyenv/versions/3.9.1/lib/python3.9/gzip.py:173: FileNotFoundError

[codecov-commenter]

Codecov Report

Merging #2934 (9f64f0f) into master (32c0d96) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #2934      +/-   ##
==========================================
- Coverage   84.66%   84.60%   -0.06%     
==========================================
  Files         607      608       +1     
  Lines       44759    44787      +28     
==========================================
  Hits        37894    37894              
- Misses       6865     6893      +28     

Impacted Files	Coverage Δ
tests/profiling/_test_multiprocessing.py	0.00% <0.00%> (ø)
tests/profiling/test_main.py	0.00% <0.00%> (ø)

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