Maintenance: remove some unit test headers (#1293)

Maintenance: remove some unit test headers (#1293)

When a unit test class is not reused across translation units, merge the
class declaration with its implementation and remove the header file. In
many cases, these classes are not meant/designed to be reusable that
way, and we should not imply otherwise by publishing a header file.

Excuse src/tests/testAuth.h until we make that test work again.