tests: fix a few fd leaks

We relied on the finalizer to close a few fds.
For some reason, this did not cause problems on Linux,
but on MacOS, it causes unmount failures:

umount(/private/tmp/gocryptfs-test-parent/194654785/default-plain): Resource busy -- try 'diskutil unmount'

tests/defaults/diriv_test.go

@@ -45,7 +45,7 @@ func TestDirIVRace(t *testing.T) {
 		for {
 			// Keep dir2 in the diriv cache
 			fd, err2 := os.Open(file2)
-			if err2 != nil {
+			if err2 == nil {
 				fd.Close()
 			}
 			if stop {

tests/defaults/main_test.go

@@ -51,6 +51,7 @@ func TestOpenTruncateRead(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer writeFd.Close()
 	abc := []byte("abc")
 	_, err = writeFd.WriteAt(abc, 0)
 	if err != nil {
@@ -61,6 +62,7 @@ func TestOpenTruncateRead(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	defer readFd.Close()
 	content := make([]byte, 3)
 	_, err = readFd.ReadAt(content, 0)
 	if err != nil {

tests/matrix/matrix_test.go

@@ -405,10 +405,12 @@ func TestRmwRace(t *testing.T) {
 	if err != nil {
 		t.Fatalf("file create failed")
 	}
+	defer f1.Close()
 	f2, err := os.Create(fn)
 	if err != nil {
 		t.Fatalf("file create failed")
 	}
+	defer f2.Close()
 
 	oldBlock := bytes.Repeat([]byte("o"), 4096)