We rarely use the same configuration in multiple test classes, but
Spring’s Test framework caches each context by default. For projects
with large numbers of integration tests, this can lead to tens of
contexts being cached. This increases memory usage, live thread count,
etc for no benefit.

This commit adds @DirtiesContext to the integration tests in
spring-boot, spring-boot-autoconfigure, and spring-boot-actuator so
that the context is closed once the test class has completed.

See gh-5141