test: do not open fixture files for writing

Use temp directory for open with `O_DSYNC` (which indicates a write may
occur) rather than `fixtures` directory. Additionally, test can be run
on macOS so allow that in addition to Linux.

PR-URL: #17810
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: Trott

test/parallel/test-fs-open-flags.js

@@ -22,10 +22,12 @@
 // Flags: --expose_internals
 'use strict';
 const common = require('../common');
+
 const fixtures = require('../common/fixtures');
-const assert = require('assert');
 
+const assert = require('assert');
 const fs = require('fs');
+const path = require('path');
 
 const O_APPEND = fs.constants.O_APPEND || 0;
 const O_CREAT = fs.constants.O_CREAT || 0;
@@ -81,8 +83,9 @@ common.expectsError(
   { code: 'ERR_INVALID_OPT_VALUE', type: TypeError }
 );
 
-if (common.isLinux) {
-  const file = fixtures.path('a.js');
-
+if (common.isLinux || common.isOSX) {
+  common.refreshTmpDir();
+  const file = path.join(common.tmpDir, 'a.js');
+  fs.copyFileSync(fixtures.path('a.js'), file);
   fs.open(file, O_DSYNC, common.mustCall(assert.ifError));
 }