encoding/jsonschema: add a test for panicking about invalid pos

This CL adds a test for the case that will be fixed in the next CL.

Change-Id: Idfa5d84ede406058d18ae33a5765fdd7da8d5d00
Signed-off-by: haoqixu <hq.xu0o0@gmail.com>
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1204320
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cmd/cue/cmd/testdata/script/invalid_tag_pos.txtar

@@ -0,0 +1,9 @@
+! exec cue def jsonschema: schema.json
+stderr 'panic: illegal Pos value'
+
+-- schema.json --
+{
+  "$id": "https://2.test.example/object                                                                                                                                                                   ",
+  "type": "object",
+  "additionalProperties": false
+}