Check stderr for regex matches, as well as stdout to fix failing test

internal/cmdtest/test_cmd.go

@@ -147,11 +147,20 @@ func (tt *TestCmd) matchExactOutput(want []byte) error {
 // after ExpectRegexp.
 func (tt *TestCmd) ExpectRegexp(regex string) (*regexp.Regexp, []string) {
 	regex = strings.TrimPrefix(regex, "\n")
+
 	var (
 		re      = regexp.MustCompile(regex)
 		rtee    = &runeTee{in: tt.stdout}
 		matches []int
 	)
+
+	// Read stderr into stdout so that its output is checked for matches, too
+	_, err := tt.stdout.Read(tt.stderr.buf.Bytes())
+	if err != nil {
+		tt.Fatalf("could not read stderr into stdout: %v", err)
+		return re, nil
+	}
+
 	tt.withKillTimeout(func() { matches = re.FindReaderSubmatchIndex(rtee) })
 	output := rtee.buf.Bytes()
 	if matches == nil {