Fix failing mochitests (#5676)

Author: jasonLaster

src/test/mochitest/browser_dbg-pretty-print-console.js

@@ -23,6 +23,8 @@ add_task(async function() {
   info("Switch back to debugger and pretty-print");
   await toolbox.selectTool("jsdebugger");
   await selectSource(dbg, "math.min.js", 2);
+  await waitForSelectedSource(dbg, "math.min.js");
+
   clickElement(dbg, "prettyPrintButton");
 
   await waitForSource(dbg, "math.min.js:formatted");

src/test/mochitest/browser_dbg-sourcemaps2.js

@@ -45,6 +45,8 @@ add_task(async function() {
   // Tests the existence of the sourcemap link in the original source.
   ok(findElement(dbg, "sourceMapLink"), "Sourcemap link in original source");
   await selectSource(dbg, "main.min.js");
+  await waitForSelectedSource(dbg, "main.min.js");
+
   ok(
     !findElement(dbg, "sourceMapLink"),
     "No Sourcemap link exists in generated source"