Rename `ContextLoader` to `ApplicationContextTester` and provide
distinct subclasses for standard, web and reactive application contexts.
Context callbacks now return AssertJ compatible contexts, allowing
tests to run directly on context. For example:
context.run((loaded) -> assertThat(loaded).hasBean("foo"));
The returned context can now also represent a context that has failed
to start (meaning that the `loadAndFail` methods are no longer needed):
context.run((loaded) -> assertThat(loaded).hasFailed());
Configuration classes are loaded via the recently introduced
`Configurations` class. This means that the tester no longer needs to
be directly aware of auto-configuration concepts.
See gh-9634
24d086066b