Reinject Servlet mocks between TestNG test methods

Prior to this commit, if multiple test methods were executed in a
subclass of AbstractTestNGSpringContextTests annotated with
@WebAppConfiguration, then injected Servlet API mocks would only
reference the mocks created for the first test method. Subsequent test
methods could therefore never reference the current mocks, and there
was a discrepancy between the state of the injected mocks and the mock
set in the RequestContextHolder.

This commit addresses this issue by ensuring that dependencies
(including updated mocks) are injected into the test instance before
the next test method if the ServletTestExecutionListener resets the
request attributes in RequestContextHolder.

Issue: SPR-11626