Include checkerMain and checkerEntrypoints in the coverage report

[gkaracha]

Closes #163.

I got a little confused by this one, since checkerMain.ml and checkerEntrypoints.ml are both already listed in src/dune after all. Apparently bisect_ppx is a little lazy: having src/checkerEntrypoints.ml and src/checkerMain.ml around is not enough, but the testsuite must have some dependency on them for them to be included on the report. Simply opening them doesn't do it either. So, this PR:

• Adds basic tests for CheckerMain.initial_wrapper, to ensure that checkerMain.ml and checkerEntrypoints.ml are included in the test coverage report.
• Makes make test-coverage have an explicit dependency on src/checkerEntrypoints.ml, since the file is now needed for instrumentation.

[gkaracha]

Locally I witness a test coverage reduction from 89.33% 83.25%.

[gkaracha]

Thanks for the review! :pray: