encoding/jsonschema: add test cases for nested values

The current `state.value` logic looks wrong: it sets the `allowedTypes`
value recursively when it encounters a structured value such
as an array or an object even though only the top level
of the value should influence the allowed types.

As it happens, the fact that `usedTypes` is set at the same
time hides this error, and it _seems_ that no other
test fails as a result. However, upcoming changes will reveal
this shortcoming, so we add a couple of test cases to
show the fix.

Signed-off-by: Roger Peppe <[email protected]>
Change-Id: I85173a321d3578094fc96f5b56ed6269c496c18c
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199399
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

encoding/jsonschema/testdata/constarray.txtar

@@ -0,0 +1,10 @@
+-- type.json --
+{
+    "type": "array",
+    "const": [
+        1
+    ]
+}
+
+-- out/decode/cue --
+[1]

encoding/jsonschema/testdata/constobj.txtar

@@ -0,0 +1,10 @@
+
+-- type.json --
+{
+    "type": "object",
+    "const": {"a": 1}
+}
+
+-- out/decode/cue --
+a: 1
+...