Bug 1411654 - Pre: Clear icon disk storage in test. r=mcomella

Newer versions of Robolectric seem to have different semantics about
clearing disk caches, so this is necessary.  But for older versions,
it shouldn't hurt, and is slightly more clear than relying on an
implicit clear.

MozReview-Commit-ID: LRcaEPasXj8

--HG--
extra : rebase_source : 3b26f65d455c049b6190a9c481f8a4bec4e06dfd

mobile/android/app/src/test/java/org/mozilla/gecko/icons/preparation/TestLookupIconUrl.java

@@ -28,6 +28,7 @@ public class TestLookupIconUrl {
     @Before
     public void setUp() {
         MemoryStorage.get().evictAll();
+        DiskStorage.get(RuntimeEnvironment.application).evictAll();
     }
 
     @Test