Integrate with @TestPropertySource

Spring 4.2 has a @TestPropertySource which has some of the features of
@IntegrationTest. This change adds @TestPropertySource to the @IntegrationTest
annotation, so that (for instance) the cache key for the context includes
properties for the test.

Since @IntegrationTest has slightly different semantics I do not propose to
deprecate it. Users can use it or @TestPropertySource, the main difference being that
with @IntegrationTest the Spring Boot context loader is aware of the annotation
and it will set sensible defaults for server.port and spring.jmx.enabled.

There are some reflection hacks to overcome the usual fortifications of Spring Test.

Fixes gh-1697