Support custom properties file formats in @TestPropertySource

Spring Framework 4.3 introduced the `PropertySourceFactory` SPI for use
with `@PropertySource` on `@Configuration` classes; however, prior to
this commit there was no mechanism to support custom properties file
formats in `@TestPropertySource` for integration tests.

This commit introduces support for configuring a custom
`PropertySourceFactory` via a new `factory` attribute in
`@TestPropertySource` in order to support custom file formats such as
JSON, YAML, etc.

For example, if you create a YamlPropertySourceFactory, you can use it
in integration tests as follows.

@SpringJUnitConfig
@TestPropertySource(locations = "/test.yaml", factory = YamlPropertySourceFactory.class)
class MyTestClass { /* ... /* }

If a custom factory is not specified, traditional `*.properties` and
`*.xml` based `java.util.Properties` file formats are supported, which
was the existing behavior.

Closes gh-30981

spring-test/src/test/resources/org/springframework/test/context/env/repeatable/local.yaml

@@ -0,0 +1,5 @@
+key1: local file
+environments:
+  dev:
+    url: https://dev.example.com
+    name: Developer Setup

spring-test/src/test/resources/org/springframework/test/context/env/test-properties.yaml

@@ -0,0 +1,7 @@
+environments:
+  dev:
+    url: https://dev.example.com
+    name: Developer Setup
+  prod:
+    url: https://prod.example.com
+    name: My Cool App