Run code coverage on v2 unit tests. (#9919)

### Problem

V2 pytest code coverage doesn't have enough tests so it is fragile.

### Solution

Long term we should have tests, but for now, just enabling it and dogfooding it will expose issues.

### Result

Helps to prevent regressions like: #9914. #10299 follows up to send this somewhere.

pants.travis-ci.toml

@@ -20,3 +20,9 @@ process_execution_local_parallelism = "%(travis_parallelism)s"
 [python-setup]
 # TODO: See https://github.com/pantsbuild/pants/issues/9964
 resolver_jobs = 2
+
+[test]
+use_coverage = true
+
+[coverage-py]
+report=["raw"]