Add YamlPropertySourceFactory that can be used for loading YAML files through the @TestPropertySource annotation

[nosan]

I have prepared two potential solutions for this #33434 enhancement.

The first approach offers a simple and intuitive way to use it:

@SpringJUnitConfig
@TestPropertySource(locations = "classpath:test.yaml", factory = YamlPropertySourceFactory.class)
class SomeTest {

}

YamlPropertySourceFactory loads all properties from the specified YAML file using YamlPropertySourceLoader and returns a CompositePropertySource containing the loaded properties.

As a potential improvement, a dedicated annotation like @TestYamlPropertySource could be introduced to simplify the loading of YAML files in tests.

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@TestPropertySource(factory = YamlPropertySourceFactory.class)
@Repeatable(TestYamlPropertySources.class)
public @interface TestYamlPropertySource {

	@AliasFor(attribute = "value", annotation = TestPropertySource.class)
	String[] value() default {};

	@AliasFor(attribute = "locations", annotation = TestPropertySource.class)
	String[] locations() default {};

	@AliasFor(attribute = "inheritLocations", annotation = TestPropertySource.class)
	boolean inheritLocations() default true;

	@AliasFor(attribute = "properties", annotation = TestPropertySource.class)
	String[] properties() default {};

	@AliasFor(attribute = "inheritProperties", annotation = TestPropertySource.class)
	boolean inheritProperties() default true;

}

With a dedicated annotation, the following syntax could be supported:

@SpringJUnitConfig
@TestYamlPropertySource({ "classpath:test.yaml", "classpath:test1.yaml" })
@TestYamlPropertySource(locations = "classpath:test2.yaml", properties = "key:value")
class TestYamlPropertySourceIntegrationTests {

}

---

An alternative approach is to customize PropertySourceDescriptor in SpringBootTestContextBootstrapper by using the PropertySourceLoaderPropertySourceFactory, which leverages the existing PropertySourceLoader. With that approach, the following syntax would be possible:

@SpringBootTest
@TestPropertySource(locations = "classpath:test.yaml")
class SomeTest {
	
}

But it has several disadvantages:

• It works only with the @SpringBootTest annotation. For @SpringJUnitConfig, the factory needs to be explicitly used.

• It does not support EncodedResource for .properties files because PropertySourceLoader only works with Resource objects. As a result, if someone is using @TestPropertySource with the encoding attribute, these changes could potentially impact them.

• Not obvious.

The first approach is much better and straightforward, and people would not have any problem using it.

[nosan]

The third option
is similar to the first approach, but instead of supporting only .yaml files, it also
supports multi-document .properties.

@SpringJUnitConfig
@TestPropertySource(locations = { "classpath:test.yaml", "classpath:test1.yaml", "classpath:test.properties" },
		factory = PropertySourceLoaderPropertySourceFactory.class)
class PropertySourceLoaderPropertySourceFactoryIntegrationTests {

}