all: add tests to cover HTML escaping via encoding/json

Note how the HTML escaping always happens, even when cmd/cue's --escape
flag or the json.HTMLEscape standard library APIs are not used.
This is the bug that the next CL is going to fix, updating the tests.

For #1243.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Iff6cf70e2ce7b5a6f9d7cbf14f3edf390b222e6f
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/552319
Unity-Result: CUEcueckoo <cueckoo@cuelang.org>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

cmd/cue/cmd/testdata/script/export_escape.txtar

@@ -0,0 +1,26 @@
+# Verify that export with and without --escape works as expected.
+
+exec cue export --out json file.cue
+cmp stdout stdout.golden
+
+exec cue export --out json --escape file.cue
+cmp stdout stdout-escape.golden
+
+-- file.cue --
+package hello
+
+simple: "hello"
+specialJSON: #"\ ""#
+specialHTML: "& < >"
+-- stdout.golden --
+{
+    "simple": "hello",
+    "specialJSON": "\\ \"",
+    "specialHTML": "\u0026 \u003c \u003e"
+}
+-- stdout-escape.golden --
+{
+    "simple": "hello",
+    "specialJSON": "\\ \"",
+    "specialHTML": "\u0026 \u003c \u003e"
+}

pkg/encoding/json/testdata/gen.txtar

@@ -15,6 +15,10 @@ t8: {
 	y: json.Marshal({a: x})
 }
 t9: json.MarshalStream([{a: 1}, {b: int | *2}])
+t10: json.Marshal({a: #"\ " & < >"#})
+t11: json.HTMLEscape(t10)
+t12: json.MarshalStream([{a: #"\ " & < >"#}, {b: ""}])
+t13: json.HTMLEscape(t12)
 
 unmarshalStream: {
 	t1:    json.UnmarshalStream(#"{"a": 1}{"b": 2}"#)
@@ -60,6 +64,18 @@ t9: """
 	{"a":1}
 	{"b":2}
 
+	"""
+t10: "{\"a\":\"\\\\ \\\" \\u0026 \\u003c \\u003e\"}"
+t11: "{\"a\":\"\\\\ \\\" \\u0026 \\u003c \\u003e\"}"
+t12: """
+	{"a":"\\\\ \\" \\u0026 \\u003c \\u003e"}
+	{"b":""}
+
+	"""
+t13: """
+	{"a":"\\\\ \\" \\u0026 \\u003c \\u003e"}
+	{"b":""}
+
 	"""
 unmarshalStream: {
 	t1: [{