internal/cuetxtar: Be noisier if DebugArchive is faulty

If DebugArchive was provided, but was not a valid txtar archive, the
cuetxtar code would, via some code paths, silently skip the test.

This change makes sure that if DebugArchive is provided but seems to
contain no files, the test is fatally aborted.

Signed-off-by: Matthew Sackman <[email protected]>
Change-Id: I9c17d8c642b107c3a4dff68df4afda340b283582
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1198553
Reviewed-by: Daniel Martí <[email protected]>
TryBot-Result: CUEcueckoo <[email protected]>

Author: cuematthew

internal/cuetxtar/txtar.go

@@ -361,6 +361,9 @@ func (x *TxTarTest) run(t *testing.T, f func(tc *Test)) {
 		archive := txtar.Parse([]byte(x.DebugArchive))
 
 		t.Run("", func(t *testing.T) {
+			if len(archive.Files) == 0 {
+				t.Fatal("DebugArchive contained no files")
+			}
 			tc := &Test{
 				T:       t,
 				Archive: archive,