This commit adds ContextLoader, a test helper that configures an
ApplicationContext that is meant to simulate a particular
auto-configuration scenario.
The auto-configuration, user configuration and environment can be
customized. The loader invokes a ContextConsumer to assert the context
and automatically close the context once it is done.
Concretely, tests can create a shared field instance of that helper with
the shared configuration to increase its visibility and tune the context
further in each test.
If the context is expected to fail, `loadAndFail` allows to optionally
assert the root exception and consume it for further assertions.
This commit also migrates some tests to illustrate the practical use of
the helper
Closes gh-9634