In order to simplify configuration of the SpringMethodRule and to ensure
that the correct TestContextManager is always retrieved for the
currently executing test class, this commit introduces a static
TestContextManager cache in SpringClassRule.
In addition, since it is not foreseen that SpringClassRule and
SpringMethodRule should be able to be subclassed, their internal methods
are now private instead of protected.
Issue: SPR-7731
973582e7df