tests: detect and report chmod failures earlier

Instead of reporting the consequence:

    matrix_test.go:906: modeHave 0664 != modeWant 0777

Report it if chmod itself fails, and also report the old file mode:

    matrix_test.go:901: chmod 000 -> 777 failed: bad file descriptor

1 files changed, 9 insertions, 1 deletions

diff --git a/tests/matrix/matrix_test.go b/tests/matrix/matrix_test.go
index 4938ba9..6ec41bd 100644
--- a/tests/matrix/matrix_test.go
+++ b/tests/matrix/matrix_test.go
@@ -896,11 +896,19 @@ func TestChmod(t *testing.T) {
 	file.Close()
 	modes := []os.FileMode{0777, 0707, 0606, 0666, 0444, 0000, 0111, 0123, 0321}
 	for _, modeWant := range modes {
-		os.Chmod(path, modeWant)
 		fi, err := os.Stat(path)
 		if err != nil {
 			t.Fatal(err)
 		}
+		err = syscall.Chmod(path, uint32(modeWant))
+		if err != nil {
+			t.Errorf("chmod %03o -> %03o failed: %v", fi.Mode(), modeWant, err)
+			continue
+		}
+		fi, err = os.Stat(path)
+		if err != nil {
+			t.Fatal(err)
+		}
 		modeHave := fi.Mode()
 		if modeHave != modeWant {
 			t.Errorf("modeHave %#o != modeWant %#o", modeHave, modeWant)