Provide the default lcov_merger to all Starlark tests

[fmeum]

Since 5b216b2, all Starlark-defined test rules either need to set the _lcov_merger attribute or fail if run with bazel coverage. This is fixed by attaching the default lcov merger as a late-bound default to all Starlark rules via a new attribute and falling back to that attribute in the TestActionBuilder.

This commit ensures backwards compatibility is maintained with existing Starlark test rules that set the _lcov_merger attribute. This is also verified in the included integration test.

Fixes #13978.

[fmeum]

@oquenchil Would you be able to take a look? The issue this resolves is labeled with team-Rules-CPP.

[meteorcloudy]

@c-mita Can you help review this change?

[c-mita]

Unfortunately, I can't accept this PR as is.
Internally, things are setup a little differently for coverage collection. We use a different collect_coverage.sh script and generally do not set the :lcov_attribute; this change does not play well with the internal setup.

I think the best (only?) way to handle this at the moment is by changing Bazel's collect_coverage.sh to simply do nothing if the LCOV_MERGER variable has not been set.

[fmeum]

@c-mita I see the problem. While your proposed solution does resolve the linked issue, it would mean that there would still be no way for Starlark tests to access the merge as a late-bound attribute.

How about the following path forward:

1. You make the required change to collect_coverage.sh to resolve the release blocking Custom test rule without _lcov_merger attribute always fails under bazel coverage #13978 and I close the current PR.
2. Someone who is familiar with Google's internal coverage setup provides some guidance on either Expose CoverageOutputGenerator's label in a configuration #8736 or _lcov_merger late-bound test attribute #10642 on how a solution to these issues could look like that would have a chance to be accepted.
3. I will look into providing such a solution as a separate PR.

[c-mita]

I'm ok with this approach; I've got a change ready to unblock the build.
It isn't ideal, but it's a way forward.