cmd/cue: add a regression test for an export -e bug fixed by evalv3

The test case added below worked on CUE v0.5, was broken by v0.6,
and luckily is fixed by the new evaluator. Add a regression test
to ensure that it keeps working.

Unlike other evaluator regression tests, this one concerns the use
of the --expression flag, so we use a testscript.

Closes #2542.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I94d45fc89251ac390c4edecc9015147f5a7f78e5
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1206351
Reviewed-by: Marcel van Lohuizen <mpvl@gmail.com>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>

cmd/cue/cmd/testdata/script/export_expr.txtar

@@ -1,10 +1,40 @@
+cd simple
 exec cue export data.cue -e a+c -e d.e.f --out yaml
 cmp stdout expect-stdout
--- expect-stdout --
+cd ..
+
+# Issue 2542 was a regression introduced in CUE v0.6 which was resolved by evalv3.
+env CUE_EXPERIMENT=evalv3
+cd issue2542
+exec cue export -e 'obj2.name' input.cue
+cmp stdout stdout.golden
+cd ..
+
+-- simple/expect-stdout --
 4
 ---
 jam
--- data.cue --
+-- simple/data.cue --
 a: 1
 c: 3
 d: e: f: "jam"
+-- issue2542/input.cue --
+obj: this={
+	(#makeName & {enable: this._enable}).out
+}
+
+obj: _enable: true
+
+obj2: obj
+
+#makeName: {
+	enable: bool
+	if enable {
+		out: name: "enabled"
+	}
+	if !enable {
+		out: name: "disabled"
+	}
+}
+-- issue2542/stdout.golden --
+"enabled"