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 : fee00a6a068d68e7f7978df56bcad94997d70afb

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