Add allow_failures to appveyor.yml until we sort out some of the issues

.appveyor.yml

@@ -27,6 +27,7 @@ build:
 
   project: cryptest.sln
   verbosity: minimal
+  parallel: true
 
 test_script:
 
@@ -37,3 +38,16 @@ test_script:
     cryptest.exe v
     
     cryptest.exe tv all
+
+# Right now, we have a few failures that we don't know how to workaround.
+#  - https://stackoverflow.com/questions/43441273/how-to-run-vcupgrade-before-appveyor-build
+#  - https://stackoverflow.com/questions/43423761/cant-perform-64-bit-testing-under-appveyor
+# Appveyor also complains that it can't find VS2010 tools even though the docs state they are there.
+matrix:
+  allow_failures:
+    - platform: Win64
+      configuration: Debug
+    - platform: Win64
+      configuration: Release
+    - image: Visual Studio 2010
+    - image: Visual Studio 2017