ART: Blacklist a libcore test for investigation

Blacklist dalvik.system.DexClassLoaderTest#testDexThenPathClassLoader
for investigation.

Bug: 28826195
Change-Id: Ibff00293af3c7bc272e3b419f4065c0fd8c069cf
diff --git a/tools/libcore_failures.txt b/tools/libcore_failures.txt
index f25fb98..bdcf86d 100644
--- a/tools/libcore_failures.txt
+++ b/tools/libcore_failures.txt
@@ -248,6 +248,11 @@
   names: [ "libcore.java.io.FileTest#testJavaIoTmpdirMutable" ]
 },
 {
+  description: "Investigate whether the test is making a wrong assumption with the newly enforced classpath.",
+  result: EXEC_FAILED,
+  names: ["dalvik.system.DexClassLoaderTest#testDexThenPathClassLoader"]
+},
+{
   description: "Made for extending, shouldn't be run",
   result: EXEC_FAILED,
   names: ["jsr166.CollectionTest#testEmptyMeansEmpty",