Update the detection logic used in ApplicationHome to:
- Deal with `!/` elements in URLs so that `BOOT-INF/classes` packaging
  works as expected.
- Use the `start-class` when no explicit source class is provided to
  prevent accidentally picking a home next to a `spring-boot.jar` that
  happens to be on the classpath.
- Ignore search logic when running from a unit test.

Fixes gh-6129