test: do not open fixture files for writing

test-fs-fsync makes a copy of a fixture file, but then doesn't do
anything with it and instead operates on the original fixture file. This
appears to be in error, and this change fixes that.

PR-URL: nodejs#17808
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
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>

test/parallel/test-fs-fsync.js

@@ -34,7 +34,7 @@ const fileTemp = path.join(common.tmpDir, 'a.js');
 common.refreshTmpDir();
 fs.copyFileSync(fileFixture, fileTemp);
 
-fs.open(fileFixture, 'a', 0o777, common.mustCall(function(err, fd) {
+fs.open(fileTemp, 'a', 0o777, common.mustCall(function(err, fd) {
   assert.ifError(err);
 
   fs.fdatasyncSync(fd);