Move to pytest

[nth10sd]

I move entirely away from unittest and on to pytest. I also took the chance to combine the chance function - previously in a separate branch it didn't work under unittest+pytest but under only pytest, it now works great.

[codecov-io]

Codecov Report

Merging #222 into master will decrease coverage by 0.58%. The diff coverage is 98.15%.

@@            Coverage Diff             @@
##           master     #222      +/-   ##
==========================================
- Coverage    57.5%   56.91%   -0.59%     
==========================================
  Files          37       36       -1     
  Lines        2906     2857      -49     
==========================================
- Hits         1671     1626      -45     
+ Misses       1235     1231       -4

Impacted Files	Coverage Δ
tests/js/test_build_options.py	100% <100%> (+5.26%)	:arrow_up:
tests/util/test_sm_compile_helpers.py	100% <100%> (ø)	:arrow_up:
tests/js/test_with_binaryen.py	100% <100%> (ø)	:arrow_up:
tests/test_run_ccoverage.py	100% <100%> (ø)	:arrow_up:
tests/js/test_link_fuzzer.py	100% <100%> (ø)	:arrow_up:
tests/util/test_fork_join.py	100% <100%> (ø)	:arrow_up:
tests/util/test_os_ops.py	100% <100%> (ø)	:arrow_up:
tests/util/test_file_system_helpers.py	100% <100%> (ø)	:arrow_up:
src/funfuzz/js/shell_flags.py	95.94% <100%> (-0.03%)	:arrow_down:
src/funfuzz/js/build_options.py	63.29% <100%> (+0.53%)	:arrow_up:
... and 6 more

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