encoding/jsonschema: add test case for issue 3176

The current logic gets the disjunction wrong when
generating CUE for jsonschema when there's a
oneOf that is then modified by an additional constraint.
Specifically the resulting schema does not allow
a string value when it should.

This CL adds a test case that will be fixed in a subsequent CL.

For #3176.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: I96a24d981fb4a90c55c9f09bee6db1bbde047648
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1199400
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

encoding/jsonschema/testdata/issue3176.txtar

@@ -0,0 +1,26 @@
+-- definition.json --
+{
+	"oneOf": [
+		{
+			"type": "object"
+		},
+		{
+			"maxLength": 3,
+			"type": "string"
+		}
+	],
+	"patternProperties": {
+		"^x-": {
+			"type": "string"
+		}
+	}
+}
+-- out/decode/cue --
+import "strings"
+
+({
+	...
+} | strings.MaxRunes(3)) & {
+	{[=~"^x-" & !~"^()$"]: string}
+	...
+}