Disable flaky test.java.lang.invoke.MethodTypeTest tests

vogar may not properly support @BeforeMethod and @AfterMethod
annotations so testGeneric() and testMakeGeneric() are flaky on
art-buildbot.

This CL disables these tests before vogar gets a fix which is tracked by
http://b/231292654.

Bug: 231292654
Bug: 231283845
Test: art/tools/run-libcore-tests.sh --mode=host -- \
      test.java.lang.invoke.MethodTypeTest
Change-Id: I03efc5591417825a4afabc525f988fb0c36d8187
diff --git a/tools/libcore_failures.txt b/tools/libcore_failures.txt
index 6e6ccb8..c6d2506 100644
--- a/tools/libcore_failures.txt
+++ b/tools/libcore_failures.txt
@@ -330,5 +330,12 @@
   bug: 228441328,
   names: ["tck.java.time",
           "test.java.time"]
+},
+{
+  description: "vogar does not respect '@BeforeMethod' and '@AfterMethod' annotations yet",
+  result: ERROR,
+  bug: 231283845,
+  names: ["test.java.lang.invoke.MethodTypeTest#testGeneric",
+          "test.java.lang.invoke.MethodTypeTest#testMakeGeneric"]
 }
 ]