ART: Ensure SlowDebug is enabled in all gtests

This was missed when moving to common_art_test.

Test: m test-art-gtest
Change-Id: I0ff50983cf71bd86b32350222caac2a36a72ad4b
diff --git a/libartbase/base/common_art_test.cc b/libartbase/base/common_art_test.cc
index be1e832..e2e85f2 100644
--- a/libartbase/base/common_art_test.cc
+++ b/libartbase/base/common_art_test.cc
@@ -225,6 +225,11 @@
   dalvik_cache_.append("/dalvik-cache");
   int mkdir_result = mkdir(dalvik_cache_.c_str(), 0700);
   ASSERT_EQ(mkdir_result, 0);
+
+  static bool gSlowDebugTestFlag = false;
+  RegisterRuntimeDebugFlag(&gSlowDebugTestFlag);
+  SetRuntimeDebugFlagsEnabled(true);
+  CHECK(gSlowDebugTestFlag);
 }
 
 void CommonArtTestImpl::TearDownAndroidDataDir(const std::string& android_data,