Create basic unit tests for library model generation

[msridhar]

This is in preparation for #1006. When working on that PR I realized we really need some unit tests to more easily debug various scenarios (right now all we have are integration tests). Plus this will make our code coverage reports more useful. This adds some infrastructure for unit tests and a couple of basic tests; we will add more as we go. Also fix a couple minor issues in LibraryModelGenerator.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.

Project coverage is 87.28%. Comparing base (b7756ea) to head (fdd57b6).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
.../uber/nullaway/libmodel/LibraryModelGenerator.java	90.47%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1031      +/-   ##
============================================
+ Coverage     86.06%   87.28%   +1.21%     
- Complexity     2110     2117       +7     
============================================
  Files            83       83              
  Lines          6932     6943      +11     
  Branches       1344     1346       +2     
============================================
+ Hits           5966     6060      +94     
+ Misses          550      456      -94     
- Partials        416      427      +11     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.