Honor presence of @WebAppConfiguration in ServTEL

The previous commit for issue SPR-11144 revealed a bug in
ServletTestExecutionListener (STEL). Specifically, STEL acted on the
fact that the ApplicationContext for a given TestContext was an
instance of WebApplicationContext. This behavior could potentially
break test code from previous releases of the Spring Framework that
relied on a custom setup of the RequestAttributes in the
RequestContextHolder with a custom WebApplicationContext ContextLoader.

This commit addresses this issue by ensuring that STEL only comes into
play if the test class is annotated with @WebAppConfiguration (for
prepareTestInstance() and beforeTestMethod()) or if the TestContext
attribute named RESET_REQUEST_CONTEXT_HOLDER_ATTRIBUTE is set to
Boolean.TRUE (for afterTestMethod()).

Issue: SPR-11144