Two small fixes in db_test

Summary:
Two fixes:
(1) WalDir to pick a directory under TmpDir to allow two tests running in parallel without impacting each other
(2) kBlockBasedTableWithWholeKeyHashIndex is disabled by mistake (I assume). Enable it.

Test Plan: ./db_test

Reviewers: yhchiang, ljin

Reviewed By: ljin

Subscribers: nkg-, igor, dhruba, haobo, leveldb

Differential Revision: https://reviews.facebook.net/D19389

db/db_test.cc

@@ -294,8 +294,8 @@ class DBTest {
  protected:
   // Sequence of option configurations to try
   enum OptionConfig {
-    kBlockBasedTableWithWholeKeyHashIndex,
     kDefault,
+    kBlockBasedTableWithWholeKeyHashIndex,
     kBlockBasedTableWithPrefixHashIndex,
     kPlainTableFirstBytePrefix,
     kPlainTableAllBytesPrefix,
@@ -467,7 +467,7 @@ class DBTest {
         options.db_log_dir = test::TmpDir();
         break;
       case kWalDir:
-        options.wal_dir = "/tmp/wal";
+        options.wal_dir = test::TmpDir() + "/wal";
         break;
       case kManifestFileSize:
         options.max_manifest_file_size = 50; // 50 bytes