Introduce @TestPropertySource support in the TCF

Spring Framework 3.1 introduced an Environment abstraction with support
for hierarchical PropertySources that can be configured
programmatically as well as declaratively via the @PropertySource
annotation. However, prior to this commit, there was no way to
declaratively configure PropertySources in integration tests in the
Spring TestContext Framework (TCF).

This commit introduces declarative support for PropertySources in the
TCF via a new class-level @TestPropertySource annotation. This
annotation provides two options for declaring test property sources:

 - The 'locations' attribute allows developers to declare external
   resource locations for test properties files.

 - The 'properties' attribute allows developers to declare inlined
   properties in the form of key-value pairs.

Test properties files are added to the Environment before all other
property sources and can therefore override system and application
property sources. Similarly, inlined properties are added to the
Environment before all other property sources and can therefore
override system property sources, application property sources, and
test properties files.

Specifically, this commit introduces the following major changes:

 - Introduced @TestPropertySource annotation along with internal
   TestPropertySourceAttributes, MergedTestPropertySources, and
   TestPropertySourceUtils for working with test property sources
   within the TCF.

 - All TestContextBootstrappers have been modified to support the
   merged property resource locations and inlined properties from
   @TestPropertySource.

 - MergedContextConfiguration (and consequently the context caching
   key) is now additionally based on the merged property resource
   locations and inlined properties from @TestPropertySource. The same
   applies to WebMergedContextConfiguration.

 - AbstractContextLoader's prepareContext() method now adds
   PropertySources for all resource locations and inlined properties
   from the supplied MergedContextConfiguration to the Environment of
   the supplied ApplicationContext. All subclasses of
   AbstractGenericContextLoader and AbstractGenericWebContextLoader
   therefore automatically provide support for @TestPropertySource.

Issue: SPR-12051