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.
This adds new tests to the TestNormalize, TestMul, TestAdd2 functions
which trigger an issue with modular reduction that was fixed in the
prevous commit to prevent regressions.
