speedup 'make check'

Summary:
Make SnapshotConcurrentAccessTest run in the beginning of the queue.

Test Plan
`make all check -j64` on devserver
Closes https://github.com/facebook/rocksdb/pull/2962

Differential Revision: D5965871

Pulled By: yiwu-arbug

fbshipit-source-id: 8cb5a47c2468be0fbbb929226a143ec5848bfaa9

Makefile

@@ -742,7 +742,7 @@ gen_parallel_tests:
 # 107.816 PASS t/DBTest.EncodeDecompressedBlockSizeTest
 #
 slow_test_regexp = \
-	^t/run-table_test-HarnessTest.Randomized$$|^t/run-db_test-.*(?:FileCreationRandomFailure|EncodeDecompressedBlockSizeTest)$$|^.*RecoverFromCorruptedWALWithoutFlush$$
+	^.*SnapshotConcurrentAccessTest.*$$|^t/run-table_test-HarnessTest.Randomized$$|^t/run-db_test-.*(?:FileCreationRandomFailure|EncodeDecompressedBlockSizeTest)$$|^.*RecoverFromCorruptedWALWithoutFlush$$
 prioritize_long_running_tests =						\
   perl -pe 's,($(slow_test_regexp)),100 $$1,'				\
     | sort -k1,1gr							\