Avoid crash during test runs

In tests that have writes occuring async WRT the test loop, we sometimes crash
when the transaction ends and we end the background task, because
CurrentAppContext() is momentarily nil between test runs.

SignalServiceKit/src/Storage/OWSStorage.m

@@ -86,7 +86,7 @@ NSString *const kNSUserDefaults_DatabaseExtensionVersionMap = @"kNSUserDefaults_
     OWSAssertDebug(delegate.areAllRegistrationsComplete);
 
     OWSBackgroundTask *_Nullable backgroundTask = nil;
-    if (CurrentAppContext().isMainApp) {
+    if (CurrentAppContext().isMainApp && !CurrentAppContext().isRunningTests) {
         backgroundTask = [OWSBackgroundTask backgroundTaskWithLabelStr:__PRETTY_FUNCTION__];
     }
     [super readWriteWithBlock:block];