Add a new `useMainMethod` attribute to `SpringBootTest` which can be
used to determine how the test should run. The three available options
are:
- `ALWAYS`
- `NEVER`
- `WHEN_AVAILABLE`
The default is `WHEN_AVAILABLE` which will attempt to launch the test
using the `main` method if there is one.
The `SpringBootContextLoader` has been updated to use the new
`SpringApplicationHook` interface when the main method is being used.
Closes gh-22405
41e0bbf4bb