cmd/cue: add a test case for a subcommand typo

The next commit enables cobra's feature to suggest a command name.

Signed-off-by: Daniel Martí <mvdan@mvdan.cc>
Change-Id: Ie9d29bcb72eed7c51c86eb4f6494c3874f3e47f0
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1204873
Reviewed-by: Roger Peppe <rogpeppe@gmail.com>
TryBot-Result: CUEcueckoo <cueckoo@cuelang.org>
Unity-Result: CUE porcuepine <cue.porcuepine@gmail.com>

cmd/cue/cmd/testdata/script/unknown_args.txtar

@@ -6,7 +6,13 @@ cmp stderr unknown_cmd.out
 ! stdout .
 cmp stderr unknown_flag.out
 
+! exec cue evla
+! stdout .
+cmp stderr typo_cmd.stdout
+
 -- unknown_cmd.out --
 unknown command "unknown" for "cue"
 -- unknown_flag.out --
 unknown flag: --unknown
+-- typo_cmd.stdout --
+unknown command "evla" for "cue"