runtime: leave directory before removing it in TestDLLPreloadMitigation

Fixes #15120

Change-Id: I1d9a192ac163826bad8b46e8c0b0b9e218e69570
Reviewed-on: https://go-review.googlesource.com/21520
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/21698
Run-TryBot: Andrew Gerrand <adg@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/runtime/syscall_windows_test.go

@@ -780,6 +780,17 @@ func TestDLLPreloadMitigation(t *testing.T) {
 		t.Skip("skipping test: gcc is missing")
 	}
 
+	tmpdir, err := ioutil.TempDir("", "TestDLLPreloadMitigation")
+	if err != nil {
+		t.Fatal("TempDir failed: ", err)
+	}
+	defer func() {
+		err := os.RemoveAll(tmpdir)
+		if err != nil {
+			t.Error(err)
+		}
+	}()
+
 	dir0, err := os.Getwd()
 	if err != nil {
 		t.Fatal(err)
@@ -794,12 +805,6 @@ uintptr_t cfunc() {
    SetLastError(123);
 }
 `
-	tmpdir, err := ioutil.TempDir("", "TestDLLPreloadMitigation")
-	if err != nil {
-		t.Fatal("TempDir failed: ", err)
-	}
-	defer os.RemoveAll(tmpdir)
-
 	srcname := "nojack.c"
 	err = ioutil.WriteFile(filepath.Join(tmpdir, srcname), []byte(src), 0)
 	if err != nil {