[ios-sdk] Fix cache unit test.

Summary:
Cache unit test was not waiting for both queues to flush before attempting to use cached files, so it would experience
intermittent failures due to a race condition. Added a sync call in two places.

Test Plan:
- Ran unit tests many, many times.

Revert Plan:

Reviewers: jacl

Reviewed By: jacl

CC: msdkexp@

Differential Revision: https://phabricator.fb.com/D579077

src/tests/FBCacheTests.m

@@ -113,7 +113,7 @@ - (void)cacheIndex:(FBCacheIndex*)cacheIndex
 {
     NSString* filePath = [_dataCachePath stringByAppendingPathComponent:name];
     dispatch_async(_fileQueue, ^{
-        [[NSFileManager defaultManager] 
+        [[NSFileManager defaultManager]
             removeItemAtPath:filePath
             error:nil];
     });
@@ -237,6 +237,7 @@ - (void)testDataPersistence
 
     // Wait for the queue to finish
     dispatch_sync(cacheIndex.databaseQueue, ^{});
+    dispatch_sync(_fileQueue, ^{});
     STAssertEquals(
         cacheIndex.currentDiskUsage, 
         fileSize*numberOfFiles, 
@@ -333,10 +334,11 @@ - (void)testDeletingUsedData
     NSString *filePath = [tempFolder
         stringByAppendingPathComponent:fileName];
 
-    // Flush the write queue
+    // Flush the write queues
     dispatch_sync(cacheIndex.databaseQueue, ^{});
+    dispatch_sync(_fileQueue, ^{});
 
-    NSData *dataFromFile = [NSData 
+    NSData *dataFromFile = [NSData
         dataWithContentsOfFile:filePath
         options:NSDataReadingMappedAlways | NSDataReadingUncached 
         error:nil];
@@ -356,7 +358,7 @@ - (void)testDeletingUsedData
     [cacheIndex removeEntryForKey:@"test1"];
     dispatch_sync(cacheIndex.databaseQueue, ^{});
     dispatch_sync(_fileQueue, ^{});
-    
+
     BOOL fileExists = [[NSFileManager defaultManager] 
         fileExistsAtPath:filePath];
     STAssertFalse(fileExists, @"File wasn't deleted at %@", filePath);