Enable asan for C core

Also speed up a test, and disable some rarely touched but long running tests
diff --git a/build.json b/build.json
index 2079b2c..ff4c7fa 100644
--- a/build.json
+++ b/build.json
@@ -582,7 +582,8 @@
         "grpc",
         "gpr_test_util",
         "gpr"
-      ]
+      ],
+      "flaky": true
     },
     {
       "name": "census_statistics_multiple_writers_test",
@@ -638,7 +639,8 @@
         "grpc",
         "gpr_test_util",
         "gpr"
-      ]
+      ],
+      "flaky": true
     },
     {
       "name": "census_stats_store_test",