Improve diagnostics for invalid testGroup values

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

@@ -21,6 +21,10 @@ import java.util.EnumSet;
 import java.util.HashSet;
 import java.util.Set;
 
+import org.springframework.util.StringUtils;
+
+import static java.lang.String.*;
+
 /**
  * A test group used to limit when certain tests are run.
  *
@@ -64,8 +68,10 @@ public enum TestGroup {
 			try {
 				groups.add(valueOf(group.trim().toUpperCase()));
 			} catch (IllegalArgumentException e) {
-				throw new IllegalArgumentException("Unable to find test group '" + group.trim()
-						+ "' when parsing '" + value + "'");
+				throw new IllegalArgumentException(format(
+						"Unable to find test group '%s' when parsing testGroups value: '%s'. " +
+						"Available groups include: [%s]", group.trim(), value,
+						StringUtils.arrayToCommaDelimitedString(TestGroup.values())));
 			}
 		}
 		return groups;

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

@@ -62,7 +62,9 @@ public class TestGroupTests {
 	@Test
 	public void parseMissing() throws Exception {
 		thrown.expect(IllegalArgumentException.class);
-		thrown.expectMessage("Unable to find test group 'missing' when parsing 'performance, missing'");
+		thrown.expectMessage("Unable to find test group 'missing' when parsing " +
+				"testGroups value: 'performance, missing'. Available groups include: " +
+				"[LONG_RUNNING,PERFORMANCE,JMXMP]");
 		TestGroup.parse("performance, missing");
 	}