Proposal: Unify tests to _test packages and remove dot imports

[jchadwick-buf]

The export_test.go pattern can be used to make all of our tests use the _test pattern and ensure that the internal API surface we depend on in tests is explicitly defined. While not strictly necessary, this PR unifies tests using this strategy. Also, I ran go mod tidy and removed the one instance of dot imports (otherwise unused.)

The only package where this is somewhat questionable IMO is httplb where the test-exported surface is a bit larger than ideal, but even so, I still think it's not a bad idea to keep it accounted for.

[jhump]

I gotta be honest: I don't understand the motivation here. If tests need to use internal code, it's best to just leave the tests in the same package as code under test. I think we only need to move that sort of code to a _test package to break an import cycle.