Improve exception msg for inactive test ApplicationContext

This commit improves the exception message thrown when a test's
ApplicationContext is no longer active by explaining that the cause
may be due to parallel test execution.

Issue: SPR-5863

Author: sbrannen

spring-test/src/main/java/org/springframework/test/context/support/DefaultTestContext.java

@@ -103,8 +103,13 @@ public ApplicationContext getApplicationContext() {
 		if (context instanceof ConfigurableApplicationContext) {
 			@SuppressWarnings("resource")
 			ConfigurableApplicationContext cac = (ConfigurableApplicationContext) context;
-			Assert.state(cac.isActive(), () -> "The ApplicationContext loaded for [" + mergedContextConfiguration
-					+ "] is not active. Ensure that the context has not been closed programmatically.");
+			Assert.state(cac.isActive(), () ->
+					"The ApplicationContext loaded for [" + mergedContextConfiguration +
+					"] is not active. This may be due to one of the following reasons: " +
+					"1) the context was closed programmatically by user code; " +
+					"2) the context was closed during parallel test execution either " +
+					"according to @DirtiesContext semantics or due to automatic eviction " +
+					"from the ContextCache due to a maximum cache size policy.");
 		}
 		return context;
 	}