Fix DBCompactionTest failure with parallel L0-L1 compactions

Summary:
The test SuggestCompactRangeNoTwoLevel0Compactions in
DBCompactionTest fails when there are parallel L0-L1 compactions
taking place because the test makes sure that only one compaction
involving L0 takes place at any given time (since before having
parallel compactions this was impossible). I changed the test to only
run with DBOptions.max_subcompactions=1 so as to not hit this issue
which is not a correctness issue but just an inherent changing of
assumptions after introducing parallel compactions.

This failed after landing https://reviews.facebook.net/D43269#inline-321303
so now this should fix it

Test Plan: make all && make check

Reviewers: yhchiang, igor, anthony, noetzli, sdong

Reviewed By: sdong

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D46617

db/db_compaction_test.cc

@@ -1746,7 +1746,10 @@ TEST_P(DBCompactionTestWithParam, CompressLevelCompaction) {
 
 // This tests for a bug that could cause two level0 compactions running
 // concurrently
-TEST_P(DBCompactionTestWithParam, SuggestCompactRangeNoTwoLevel0Compactions) {
+// TODO(aekmekji): Make sure that the reason this fails when run with
+// max_subcompactions > 1 is not a correctness issue but just inherent to
+// running parallel L0-L1 compactions
+TEST_F(DBCompactionTest, SuggestCompactRangeNoTwoLevel0Compactions) {
   Options options = CurrentOptions();
   options.compaction_style = kCompactionStyleLevel;
   options.write_buffer_size = 110 << 10;
@@ -1758,7 +1761,6 @@ TEST_P(DBCompactionTestWithParam, SuggestCompactRangeNoTwoLevel0Compactions) {
   options.target_file_size_base = 98 << 10;
   options.max_write_buffer_number = 2;
   options.max_background_compactions = 2;
-  options.max_subcompactions = max_subcompactions_;
 
   DestroyAndReopen(options);