Bug 1096490 - Fix test failures in browser_devtools_api.js;r=vporof

These test failures occur when you replace the deprecated-sync-thenables in
protocol.js with Promise.jsm.

browser/devtools/framework/test/browser_devtools_api.js

@@ -76,7 +76,10 @@ function runTests1(aTab) {
 
     gDevTools.unregisterTool(toolId1);
 
-    runTests2();
+    // Wait for unregisterTool to select the next tool before calling runTests2,
+    // otherwise we will receive the wrong select event when waiting for
+    // unregisterTool to select the next tool in continueTests below.
+    toolbox.once("select", runTests2);
   });
 }