Add some additional jvmti-stress skips

These were failing with jvmti-stress. Tests 159 & 649 are failing
due to issues with dexter and test 647 was failing due to using
an unsupported classloader.

Bug: 38344137
Test: ./test/testrunner/testrunner.py --host -j40 --jvmti-stress
Change-Id: I1fea19540f498ada091fc64edcf7187b4af81f79
diff --git a/test/knownfailures.json b/test/knownfailures.json
index 366396f..4b44df7 100644
--- a/test/knownfailures.json
+++ b/test/knownfailures.json
@@ -557,6 +557,8 @@
         "tests": [
             "097-duplicate-method",
             "138-duplicate-classes-check2",
+            "159-app-image-fields",
+            "649-vdex-duplicate-method",
             "804-class-extends-itself",
             "921-hello-failure"
         ],
@@ -573,6 +575,7 @@
             "087-gc-after-link",
             "626-const-class-linking",
             "629-vdex-speed",
+            "647-jni-get-field-id",
             "944-transform-classloaders"
         ],
         "description": [