cmd/cue: test the "found packages $X and $Y in $DIR" error message

We had this test case which covered the error case, but it did not
actually lock in what the error message looks like

For #3703.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Ib9cec1969d43831e3ebe0c87de1f13497000fe29
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1214887
Unity-Result: CUE porcuepine <[email protected]>
Reviewed-by: Matthew Sackman <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: mvdan

cmd/cue/cmd/testdata/script/load_pkg.txtar

@@ -16,6 +16,7 @@ cmp stdout stdout-pkg
 
 # don't allow mixing two differently named packages
 ! exec cue eval kube.cue foo/kube2.cue
+cmpenv stderr different-packages.stderr
 
 # When passing multiple CUE files in different directories,
 # cue/load will sort the filenames so that children come later.
@@ -33,6 +34,8 @@ cmp stdout stdout-sort
 
 cd ..
 
+-- different-packages.stderr --
+found packages "kube" (kube.cue) and "kube2" (kube2.cue) in "${WORK@R}"
 -- data.cue --
 foo:3
 -- t.cue --