Putting the test code in the same package makes it easier for forks
since they don't have to change the import paths as much and it also
gets rid of the need for internal_test.go to bridge.
Also, remove the exception from the lint checks about returning the
unexported type since it is no longer required.
The benchmarks are still only compiled when running 'go test' so this has
no effect on regular usage. This is being done because benchmarks often
need access to internal state.
Normal tests are kept in a separate package since they also serve to
exercise the public API, and by intentionally making it more difficult to
reach into the internals, it helps ensure the public API is sanely usable.
Since the benchmarks can now access the internals directly, this commit
also removes the functions which exposed the internals to the test package
from internal_test.go which were only used by the benchmarks.
Also, it removes a duplicate benchmark.
