Test fails with IllegalAccessError from Data JPA's proxy creation when a new context is refreshed after a test that starts Tomcat has run

[wilkinsona]

https://stackoverflow.com/questions/37342590/java-lang-illegalaccesserror-when-making-spring-data-repo-package-local

The root cause is #2308. When the test that starts Tomcat (WebEnvironment.RANDOM_PORT or DEFINED_PORT) runs, the thread context class loader is set to Tomcat's web app class loader. As the context is cached, no clean up is performed after the tests are finished so the TCCL is left being Tomcat's web app class loader. This then breaks the proxy creation in a subsequent test that refreshes a new context.

We already hit another variant of the problem in #5141. We avoided it then using @DirtiesContext so that the context is closed when the tests are finished. That's not a general solution as, unlike, our integration tests where the configuration is rarely the same from test class to test class, tests for a user's application are likely to have common configuration making context caching worthwhile.

I can see two possible solutions:

1. Revert the changes made for Set Tomcat's WebappClassLoader as the main thread's context classloader #2308
2. Hook into the test framework so that the TCCL is configured correctly

If it's possible, 2 is the preferred option. We'd need a hook to restore the TCCL after a test class has finished and then to reinstate it before a test class runs.

[wilkinsona]

I've implemented a test execution listener that manages the TCCL but it feels like a bit of a hack. I'm going to explore a third option: a different fix for #2308 so that JNDI lookups work but without us messing around with the TCCL.