Convert TestContext to interface & default impl

Since the Spring TestContext Framework was introduced in Spring
Framework 2.5, the TestContext class has always been a public class
with package private constructors. The visibility of TestContext's
constructor and methods was intentionally limited in order to hide the
implementation details of the context cache, etc. However, this fact
has made it difficult (if not impossible) to unit test custom
TestExecutionListener implementations.

This commit addresses this issue by converting TestContext into a
public interface with a package private DefaultTestContext
implementation. This enables unit testing of any components that depend
on a TestContext (e.g., TestExecutionListeners) while at the same time
preserving the encapsulation of the inner workings of the TestContext
implementation with regard to context loading and caching.

Issue: SPR-7692