Fix flaky JWKS tests by shadowing mainThread in ThreadSwitcherShadow

Author: utkrishtsahu

auth0/src/test/java/com/auth0/android/request/internal/ThreadSwitcherShadow.java

@@ -26,4 +26,9 @@ public ThreadSwitcherShadow() {
     public void backgroundThread(Runnable runnable) {
         executor.execute(runnable);
     }
+
+    @Implementation
+    public void mainThread(Runnable runnable) {
+        executor.execute(runnable);
+    }
 }