test: update test-fs-fsync to run from temp

We copy the "a.js" file fixture to a temp directory and then we open
it with write access. This makes sure we are only writing to the
provided temp-dir and not in the fixture dir.

PR-URL: https://github.com/nodejs/node/pull/15537
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

test/parallel/test-fs-fsync.js

@@ -25,10 +25,16 @@ const assert = require('assert');
 const fixtures = require('../common/fixtures');
 
 const fs = require('fs');
+const path = require('path');
 
-const file = fixtures.path('a.js');
+const fileFixture = fixtures.path('a.js');
+const fileTemp = path.join(common.tmpDir, 'a.js');
 
-fs.open(file, 'a', 0o777, common.mustCall(function(err, fd) {
+// Copy fixtures to temp.
+common.refreshTmpDir();
+fs.copyFileSync(fileFixture, fileTemp);
+
+fs.open(fileFixture, 'a', 0o777, common.mustCall(function(err, fd) {
   assert.ifError(err);
 
   fs.fdatasyncSync(fd);