Add some tests...

Not very efficient to make this after development, but at least I can
ensure that some basic stuffs are checked.

update/update_test.go

@@ -14,6 +14,7 @@ func TestDownloadLatestRelease(t *testing.T) {
 
 	// change "exe" to /tmp/test-katenary
 	exe = "/tmp/test-katenary"
+	defer os.Remove(exe)
 
 	// Now call the CheckLatestVersion function
 	version, assets, err := CheckLatestVersion()