cmd/cue: use fmt --check in the fmt_multi testscript

I recently left a TODO there to use a future "check" or "diff" mode
so that we would no longer have to back up and restore files
to see that running fmt in two different ways gives the same result.
Now that --check is a feature, we can use it directly.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: I160bd0f9e54f12e4b3d3661b0c5beeeba9c293af
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1190094
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>

cmd/cue/cmd/testdata/script/fmt_multi.txtar

@@ -1,21 +1,13 @@
-# TODO(mvdan): once we have a "list" or "diff" fmt flag,
-# we can reuse the same files without worrying about modifying in-place.
-mkdir originals
-cp x.cue star.cue y.cue yb.cue originals/
-
-exec cue fmt .
-cmp x.cue out/x.cue
-cmp star.cue out/star-cue
-cmp y.cue out/y-cue
-cmp yb.cue out/yb-cue
-
-cp originals/x.cue originals/star.cue originals/y.cue originals/yb.cue .
-
-exec cue fmt x.cue star.cue y.cue yb.cue
-cmp x.cue out/x.cue
-cmp star.cue out/star-cue
-cmp y.cue out/y-cue
-cmp yb.cue out/yb-cue
+! exec cue fmt --check .
+cmp stdout stdout.golden
+
+! exec cue fmt --check x.cue star.cue y.cue yb.cue
+cmp stdout stdout.golden
+-- stdout.golden --
+star.cue
+x.cue
+y.cue
+yb.cue
 -- x.cue --
 // header