Throw exception if TxMgr cannot be retrieved for @Transactional test

Prior to this commit, a @Transactional integration test would silently
be executed without a transaction if the transaction manager could not
be retrieved from the application context -- for example, it no such
bean was defined or if multiple beans were present but none satisfied
the qualifier.

This commit addresses this issue by throwing an IllegalStateException
if the PlatformTransactionManager cannot be retrieved for a
@Transactional test.

Issue: SPR-13895

spring-test/src/main/java/org/springframework/test/context/transaction/TransactionalTestExecutionListener.java

@@ -186,6 +186,12 @@ public class TransactionalTestExecutionListener extends AbstractTestExecutionLis
 			}
 
 			tm = getTransactionManager(testContext, transactionAttribute.getQualifier());
+
+			if (tm == null) {
+				throw new IllegalStateException(String.format(
+					"Failed to retrieve PlatformTransactionManager for @Transactional test for test context %s.",
+					testContext));
+			}
 		}
 
 		if (tm != null) {