Improve extensibility of TestContext bootstrapping & context caching

- DefaultBootstrapContext and DefaultCacheAwareContextLoaderDelegate
   are now public classes in the 'support' subpackage.

 - Introduced getCacheAwareContextLoaderDelegate() in
   AbstractTestContextBootstrapper as an extension point for configuring
   custom ContextCache support.

 - Introduced reflection-based createBootstrapContext() utility method
   in BootstrapUtils; TestContextManager now delegates to BootstrapUtils
   in order to avoid package cycles.

 - Introduced logStatistics() method in the ContextCache API and defined
   statistics logging category as a constant.

 - DefaultCacheAwareContextLoaderDelegate now delegates to
   ContextCache.logStatistics().

Issue: SPR-12683

spring-test/src/test/java/org/springframework/test/context/BootstrapTestUtils.java

@@ -16,6 +16,8 @@
 
 package org.springframework.test.context;
 
+import org.springframework.test.context.support.DefaultBootstrapContext;
+
 /**
  * Collection of test-related utility methods for working with {@link BootstrapContext
  * BootstrapContexts} and {@link TestContextBootstrapper TestContextBootstrappers}.

spring-test/src/test/java/org/springframework/test/context/ClassLevelDirtiesContextTestNGTests.java

@@ -37,7 +37,7 @@ import org.springframework.test.context.testng.TrackingTestNGTestListener;
 import org.testng.TestNG;
 
 import static org.junit.Assert.*;
-import static org.springframework.test.context.ContextCacheTestUtils.*;
+import static org.springframework.test.context.support.ContextCacheTestUtils.*;
 
 /**
  * JUnit 4 based integration test which verifies correct {@linkplain ContextCache

spring-test/src/test/java/org/springframework/test/context/ClassLevelDirtiesContextTests.java

@@ -36,7 +36,7 @@ import org.springframework.test.context.support.DependencyInjectionTestExecution
 import org.springframework.test.context.support.DirtiesContextTestExecutionListener;
 
 import static org.junit.Assert.*;
-import static org.springframework.test.context.ContextCacheTestUtils.*;
+import static org.springframework.test.context.support.ContextCacheTestUtils.*;
 
 /**
  * JUnit 4 based integration test which verifies correct {@linkplain ContextCache

spring-test/src/test/java/org/springframework/test/context/ContextCacheTests.java

@@ -18,6 +18,7 @@ package org.springframework.test.context;
 
 import org.junit.Before;
 import org.junit.Test;
+
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.test.annotation.DirtiesContext.HierarchyMode;
@@ -26,7 +27,7 @@ import org.springframework.test.context.support.DefaultContextCache;
 import org.springframework.test.util.ReflectionTestUtils;
 
 import static org.junit.Assert.*;
-import static org.springframework.test.context.ContextCacheTestUtils.*;
+import static org.springframework.test.context.support.ContextCacheTestUtils.*;
 
 /**
  * Integration tests for verifying proper behavior of the {@link ContextCache} in

spring-test/src/test/java/org/springframework/test/context/SpringRunnerContextCacheTests.java

@@ -31,7 +31,7 @@ import org.springframework.test.context.support.DependencyInjectionTestExecution
 import org.springframework.test.context.support.DirtiesContextTestExecutionListener;
 
 import static org.junit.Assert.*;
-import static org.springframework.test.context.ContextCacheTestUtils.*;
+import static org.springframework.test.context.support.ContextCacheTestUtils.*;
 
 /**
  * JUnit 4 based unit test which verifies correct {@link ContextCache

spring-test/src/test/java/org/springframework/test/context/TestContextTestUtils.java

@@ -16,6 +16,9 @@
 
 package org.springframework.test.context;
 
+import org.springframework.test.context.support.DefaultBootstrapContext;
+import org.springframework.test.context.support.DefaultCacheAwareContextLoaderDelegate;
+
 /**
  * Collection of test-related utility methods for working with {@link TestContext TestContexts}.
  *

spring-test/src/test/java/org/springframework/test/context/support/ContextCacheTestUtils.java

@@ -14,7 +14,9 @@
  * limitations under the License.
  */
 
-package org.springframework.test.context;
+package org.springframework.test.context.support;
+
+import org.springframework.test.context.ContextCache;
 
 import static org.junit.Assert.*;