CI: enable codecov coverage collection + cleanups

[d4l3k]

• enables coverage collection for python unit tests
• uploads coverage to codecov.io
• adds badge to README
• updates README with instructions on how to install all dependencies
• fixes finder_test compatibility with pytest

Test plan:

CI -- check coverage is correctly computed

[codecov]

Codecov Report

Merging #214 (54cc105) into main (592c38b) will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #214      +/-   ##
==========================================
+ Coverage   90.90%   91.10%   +0.19%     
==========================================
  Files          97       57      -40     
  Lines        5247     2630    -2617     
==========================================
- Hits         4770     2396    -2374     
+ Misses        477      234     -243     

Impacted Files	Coverage Δ
torchx/aws/util.py	0.00% <0.00%> (ø)
torchx/cli/colors.py	60.00% <0.00%> (ø)
torchx/apps/utils/test/booth_test.py
torchx/schedulers/test/ids_test.py
torchx/cli/test/main_test.py
torchx/components/component_test_base.py
torchx/pipelines/kfp/test/suites.py
torchx/runner/test/api_test.py
torchx/cli/test/cmd_runopts_test.py
torchx/runtime/tracking/test/api_test.py
... and 38 more

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