Ensure ServTEL doesn't reset ReqAttrs by accident

Prior to this commit, the ServletTestExecutionListener did not
overwrite RequestAttributes in the RequestContextHolder if the
ApplicationContext associated with the given TestContext was not a
WebApplicationContext; however, the ServletTestExecutionListener would
clear the RequestAttributes after every test method execution,
regardless of whether the context was a WebApplicationContext or not.
This behavior breaks backwards compatibility with integration tests
that managed the RequestAttributes in RequestContextHolder themselves.

This commit addresses this issue by introducing a TestContext attribute
named RESET_REQUEST_CONTEXT_HOLDER_ATTRIBUTE in
ServletTestExecutionListener. This attribute is used internally within
ServletTestExecutionListener to ensure that the RequestContextHolder is
only cleared (i.e., reset) if the ServletTestExecutionListener actually
populated the RequestContextHolder.

Issue: SPR-11144