Collect coverage for other languages (#3287)

When using `--combined_report=lcov`, the coverage report for a `go_test`
will also include coverage for data dependencies executed by the test,
e.g. `java_binary` or `cc_binary` tools run in an integration test.

Note: This commit does *not* make it so that coverage is collected for
`go_binary`s executed by `go_test`s - Go doesn't provide exit hooks, so
this would be rather involved to implement.
6 files changed