Update .snyk confi to exclude tests and testutils

The snyk scan has started flagging some of our test utils for path
traversal because they can open arbitrary files. They are only used
during testing and should be able to open arbitrary files, so this
excludes them from the scanning.

Author: jkyros

.snyk

@@ -2,3 +2,5 @@ exclude:
   global:
     - "vendor/**"
     - "**/*_test.go"
+    - "cmd/testutil/**"  # these are utilities only used during testing
+    - "test/"  # these are tests