Allow @ContextConfiguration to be omitted

Prior to this commit, the @ContextConfiguration annotation was required
to be present even if default XML files, Groovy scripts, or
@Configuration classes were detected; however, in such cases the
@ContextConfiguration was typically declared empty and therefore
seemingly unnecessary boilerplate.

This commit permits @ContextConfiguration to be omitted whenever it can
be reasonably deduced. Consequently, integration tests such as the
following are now supported.

    @RunWith(SpringRunner.class)
    public class MyTest {

        @Autowired String myBean;

        @Test public void example() { /* ... */ }

        @Configuration
        static class Config {

            @Bean String myBean() {
                return "Hello";
            }
        }
    }

Issue: SPR-13955

src/asciidoc/whats-new.adoc

@@ -686,6 +686,8 @@ Spring 4.3 also improves the caching abstraction as follows:
 
 * The JUnit support in the _Spring TestContext Framework_ now requires JUnit 4.12 or higher.
 * New `SpringRunner` __alias__ for the `SpringJUnit4ClassRunner`.
+* An empty declaration of `@ContextConfiguration` can now be completely omitted if default
+  XML files, Groovy scripts, or `@Configuration` classes are detected.
 * `@BeforeTransaction` and `@AfterTransaction` methods are no longer required to be `public`.
 * Server-side Spring MVC Test supports expectations on response headers with multiple values.
 * Server-side Spring MVC Test parses form data request content and populates request parameters.