Add tests and run before build

[oliver3]

• Added pytest tests, we did not see a need to use a separate library for snapshot creation and file comparison
• Add test step for GitHub builds

Coverage after adding tests:

Name                      Stmts   Miss Branch BrPart  Cover
-----------------------------------------------------------
src/eml2csv/__init__.py       2      0      0      0   100%
src/eml2csv/lib.py          163      7     38      6    94%
src/eml2csv/main.py           7      7      0      0     0%
src/eml2csv/util.py          31      8     10      4    66%
tests/__init__.py             0      0      0      0   100%
tests/test_eml2csv.py        45      0      0      0   100%
-----------------------------------------------------------
TOTAL                       248     22     48     10    89%

[chrismostert]

Sweet, thanks. Tests look good.

I've added a separate github workflow which runs on commits since the existing workflow only runs when we create a release to add the binaries to that release. This ensures that we also run the test suite on individual commits