post-processor/vagrant: fix test failure on Windows

Close temporary file created in test before trying to remove it.
Fixes test failure on Windows, which cannot remove the file unless
it has been closed.

post-processor/vagrant/post-processor_test.go

@@ -122,20 +122,25 @@ func TestPostProcessorPrepare_subConfigs(t *testing.T) {
 }
 
 func TestPostProcessorPrepare_vagrantfileTemplateExists(t *testing.T) {
-	var p PostProcessor
-
 	f, err := ioutil.TempFile("", "packer")
 	if err != nil {
 		t.Fatalf("err: %s", err)
 	}
 
+	name := f.Name()
 	c := testConfig()
-	c["vagrantfile_template"] = f.Name()
+	c["vagrantfile_template"] = name
 
-	os.Remove(f.Name())
+	if err := f.Close(); err != nil {
+		t.Fatal("err: %s", err)
+	}
 
-	err = p.Configure(c)
-	if err == nil {
+	if err := os.Remove(name); err != nil {
+		t.Fatalf("err: %s", err)
+	}
+
+	var p PostProcessor
+	if err := p.Configure(c); err == nil {
 		t.Fatal("expected an error since vagrantfile_template does not exist")
 	}
 }