Explicitly set Linguist languages during tests

Locally, tests failed when these required a Gitaly server to start, 3 tests failed as the server was unable to find the Linguist colors. This is fixed by skipping autodetection, and overriding the default value.

Observed tests that failed:

rspec ./spec/gitaly/ref_service_spec.rb:11
rspec ./spec/gitaly/repository_service_spec.rb:10
rspec ./spec/gitaly/repository_service_spec.rb:16

Of these, now only the ref_service_spec.rb is still failing, but with a deadline exceeded error. This seems like the server is slow to start, somehow. Given my gut feeling says its unrelated, it will be fixed in a later commit.

cc @jacobvosmaer-gitlab