Support running tests

[bradleyfalzon]

As a user, GopherCI should replace all basic CI tasks that I might be using elsewhere, such as running tests.

All tests would run, not those covering lines of code that have changed in the PR, although that could be possible but unlikely to have real world benefits (tests should always pass).

Users may want to configure skipping tests in vendor directories.

This may be related to #7 in the generic sense.

I made a small note in #25 that there may be value re-running tests as the base branch changes, merge the branch (if possible/no conflicts), and then run the tests.