Investigate convention-based application.properties specifically for tests

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.

Start by reading the concern linked from issue 5307 and compare it with the existing application.properties and test configuration behavior described here. Define an out-of-the-box convention for test-specific overrides and determine whether a global user-specific test application.properties should also load; the work is done when both behaviors are specified and covered by the project’s tests.