Previously, when SpringApplicationContextLoader created a
SpringApplication and used it to load the test’s application context,
it relied upon SpringApplication correctly deducing the application’s
main class. This would result in the wrong class being deduced as the
application’s main method is not called so the test runner’s main method
would be found instead.

This commit updates SpringApplicationContextLoader to explicitly set
SpringApplication’s main class to be the current test’s class. While
not strictly the application’s main class, it is the next best thing
available in this situation and prevents misleading log messages and
application versions from being logged.

Fixes gh-2930