Skip a couple of libcore tests for gcstress config

Bug: 235566533
Test: art/test.py
Change-Id: Id7a895f4cdfabe53b43d1f077b32d74237118750
diff --git a/tools/run-libcore-tests.py b/tools/run-libcore-tests.py
index 72db66c..244f912 100755
--- a/tools/run-libcore-tests.py
+++ b/tools/run-libcore-tests.py
@@ -264,6 +264,8 @@
   "test.java.lang.StrictMath.HypotTests#testAgainstTranslit_shard4",
   "test.java.math.BigDecimal",
   "test.java.math.BigInteger#testConstructor",
+  "test.java.util.TestFormatter",
+  "test.java.util.Collection",
 }
 
 DISABLED_FUGU_TESTS = {