encoding/jsonschema: add test for numeric enum with type

The current logic causes an error when combining the `type`
specifier with a numeric enum.

This change adds a test that demonstrates that issue,
to be fixed in a subsequent CL.

For #3173.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Ie401aea5a165e7c6353ef12473adcc6a45ddce35
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199213
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

encoding/jsonschema/testdata/num.txtar

@@ -37,7 +37,6 @@
   },
   "additionalProperties": false
 }
-
 -- out/decode/cue --
 import (
 	"strings"

encoding/jsonschema/testdata/numericenum.txtar

@@ -0,0 +1,13 @@
+# Test that we can use a "number" type constraint with a numeric enum.
+# See issue #3173.
+
+# TODO fix this so it works.
+
+-- type.json --
+{
+    "type": "number",
+    "enum": [ 1, 2, 3]
+}
+-- out/decode/err --
+constraint not allowed because type number is excluded:
+    type.json:2:5