Bug 1571994 - Fix dbg-wasm-sourcemaps timing. r=bhackett

Differential Revision: https://phabricator.services.mozilla.com/D54803

--HG--
extra : moz-landing-system : lando

devtools/client/debugger/test/mochitest/browser_dbg-wasm-sourcemaps.js

@@ -20,6 +20,7 @@ add_task(async function() {
 
   info("Set breakpoint and reload the page.");
   ok(true, "Original sources exist");
+  await waitForSources(dbg, "fib.c");
   await selectSource(dbg, "fib.c");
   await addBreakpoint(dbg, "fib.c", 10);