Segregate tests that depend on the jmxmp: protocol

This commit introduces TestGroup#JMXMP and adds assumptions to related
tests accordingly. These tests require the jmxoptional_remote jar on the
classpath, and are run nightly in the SPR-PERF build.

Issue: SPR-8089

spring-core/src/test/java/org/springframework/tests/TestGroup.java

@@ -48,7 +48,13 @@ public enum TestGroup {
 	 * {@code StopWatch}, etc. should be considered a candidate as their successful
 	 * execution is likely to be based on events occurring within a given time window.
 	 */
-	PERFORMANCE;
+	PERFORMANCE,
+
+	/**
+	 * Tests requiring the presence of jmxremote_optional.jar in jre/lib/ext in order to
+	 * avoid "Unsupported protocol: jmxmp" errors.
+	 */
+	JMXMP;
 
 
 	/**