Investigate convention based application.properties specifically for tests

[philwebb]

Currently it can can be quite awkward to add an application.properties for tests that overrides only specific values of the one in src/main/java. Some folks resort to adding a profile specific property and enabling a profile in all their tests. Some mix .properties and .yaml so that both files load and some use import:.

It would be nice if we had an out-of-the-box solution.

In addition, we could try to address the concern mentioned in https://github.com/spring-projects/spring-boot/issues/5307#issuecomment-1783188324 and provide a way for a global user specific test application.properties file to also be loaded.