Support more aliases of "Test Plan" in commit messages

Summary: Add support for "tests", "testplan" and "tested" as alias of "Test Plan".

Test Plan: Created a diff with test plan specified in "tests".

Reviewers: csilvers, btrahan

Reviewed By: csilvers

CC: aran

Differential Revision: https://secure.phabricator.com/D2531

src/applications/differential/field/specification/testplan/DifferentialTestPlanFieldSpecification.php

@@ -89,6 +89,16 @@ final class DifferentialTestPlanFieldSpecification
     return 'Test Plan';
   }
 
+  public function getSupportedCommitMessageLabels() {
+    return array(
+      'Test Plan',
+      'Testplan',
+      'Tested',
+      'Tests',
+    );
+  }
+
+
   public function renderValueForCommitMessage($is_edit) {
     return $this->plan;
   }