encoding/jsonschema: move txtar tests into subdirectory

In order to prepare for adding the external JSON Schema test
data, we move our own test data into its own subdirectory.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I8755f20b87ecf03268216aa3bc7254f699b39787
Dispatch-Trailer: {"type":"trybot","CL":1200161,"patchset":3,"ref":"refs/changes/61/1200161/3","targetBranch":"master"}

encoding/jsonschema/decode_test.go

@@ -61,7 +61,7 @@ import (
 // The #openapi tag in the txtar header enables OpenAPI extraction mode.
 func TestDecode(t *testing.T) {
 	test := cuetxtar.TxTarTest{
-		Root:   "./testdata",
+		Root:   "./testdata/txtar",
 		Name:   "decode",
 		Matrix: cuetdtest.FullMatrix,
 	}