RFR: Add test to verify Shenandoah is not enabled by default, and enabled with the flag

[Aleksey Shipilev]

Upstream had changed the GC selection logic a bit, and it is easy to miss the code that enables the
GC: -XX:+UseShenandoahGC would be accepted, but Shenandoah would not enable! (Found this during
Epsilon merges). Let's add a simple test that verifies GC is enabled:

diff -r 58b1ec268475 test/hotspot/jtreg/gc/shenandoah/options/TestEnabled.java
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/test/hotspot/jtreg/gc/shenandoah/options/TestEnabled.java	Fri Apr 13 10:50:12 2018 +0200
@@ -0,0 +1,52 @@
+/*
+ * Copyright (c) 2017, 2018, Red Hat, Inc. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import java.lang.management.GarbageCollectorMXBean;
+import java.lang.management.ManagementFactory;
+
+/*
+ * @test TestEnabled
+ * @run main/othervm -Dexpected=false -Xmx64m                      TestEnabled
+ * @run main/othervm -Dexpected=true  -Xmx64m -XX:+UseShenandoahGC TestEnabled
+ */
+
+public class TestEnabled {
+
+  public static void main(String... args) {
+    boolean expected = Boolean.getBoolean("expected");
+    boolean actual = isEnabled();
+    if (expected != actual) {
+      throw new IllegalStateException("Error: expected = " + expected + ", actual = " + actual);
+    }
+  }
+
+  public static boolean isEnabled() {
+    for (GarbageCollectorMXBean bean : ManagementFactory.getGarbageCollectorMXBeans()) {
+      if (bean.getName().contains("Shenandoah")) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+}


Thanks,
-Aleksey