encoding/jsonschema: do not let cue/load walk up to the git root

When using `GODEBUG=gocachehash=1 go test .`, the number of testInput
lines goes down dramatically after the change, as the test cache logic
no longer needs to stat hundreds of files in parent directories.

    $ grep testInput before.txt | wc -l
    3960
    $ grep testInput after.txt | wc -l
    1291

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Icf1555286be13c40defd33889f99009bcc001e22
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1201439
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

encoding/jsonschema/internal/externaltest/tests.go

@@ -91,6 +91,9 @@ func ReadTestDir(dir string) (tests map[string][]*Schema, err error) {
 	os.Setenv("CUE_EXPERIMENT", "embed")
 	inst := load.Instances([]string{"."}, &load.Config{
 		Dir: dir,
+		// Just like in the cue/load tests, prevent Go tests from walking up to the root
+		// directory of the git repository, as that almost always causes test cache misses.
+		ModuleRoot: dir,
 	})[0]
 	if err := inst.Err; err != nil {
 		return nil, err