Spring Framework 4.1 introduced support for @TestPropertySource;
however, the utilities used to parse inlined properties and add test
property sources to the environment are currently private which
prevents reuse by third-party frameworks like Spring Boot.

This commit addresses this issue by making such utilities public.

 - TestPropertySourceUtils is now a public class.

 - Various utility methods in TestPropertySourceUtils have been made
   public.

 - addResourcePropertySourcesToEnvironment() has been renamed to
   addPropertiesFilesToEnvironment().

 - extractEnvironmentProperties() has been renamed to
   convertInlinedPropertiesToMap().

 - All public methods in TestPropertySourceUtils are now fully
   documented.

Issue: SPR-12721