Add coverage for testing in a native image

[wilkinsona]

Currently, all of the smoke tests run the tests on the JVM against an app running as a separate process (on the JVM using java -jar or as a native image). We should add something that exercises nativeTest with various things (slices, @SpringBootTest with a mock environment, @SpringBootTest with a web server, etc).

[wilkinsona]

This would have caught spring-projects/spring-boot#32848.

[mhalbritter]

I pushed a change which executes the unit tests on the JVM and in a native image. The nativeAotTests task now depends on nativeTest. I removed the useless unit tests from most of the smoke tests.

The native unit tests can be seen for example in the command-line-runner smoke test.

[wilkinsona]

I'd like to revisit this as I think it's resulted in individual smoke tests covering too many things. It feels like we might need two separate kinds of smoke test:

1. An app smoke test that runs an AOT-processed app on the JVM or in a native image and tests it
2. A test smoke test that runs AOT-processed tests on the JVM or in a native image

This separation will mean that we have four different tasks that can be run to test different things:

1. app smoke test
   a. test the app on the JVM
   b test the app in a native image
2. test smoke test
   a. Run the AOT-processed tests on the JVM
   b. Run the AOT-processed tests in a native image

These should be configured as separate tasks on CI so that the results provide as clear a picture as possible of what does and does not work.