[testjson] Exclude known invalid JSON from tests

There are invalid JSON files in themes/ that are used for unit tests, so
exclude those from the testjson.sh execution.

see #41

testjson.sh

@@ -1,3 +1,3 @@
 #!/bin/sh
 
-find themes/ -name "*.json"|xargs cat|json_verify -s
+find themes/ -name "*.json"|grep -v invalid|xargs cat|json_verify -s