@WebAppConfiguration expects a mock web environment. If it is used
in conjuction with @SpringBootTest configured with a RANDOM_PORT or
DEFINED_PORT web environment a null pointer exception occurs as an
assumption that's made by MockServerContainerContextCustomizer doesn't
hold true in a non-mock web environment.

This commit updates SpringBootTestContextBootstrap to detect the
illegal configuration combination and fail fast, advising the user
to remove @WebAppConfiguration or reconfigure @SpringBootTest.

Closes gh-6795