test: add tests for rs+, sr+ to test-fs-open-flags.js

PR-URL: https://github.com/nodejs/node/pull/10780
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

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

@@ -10,6 +10,7 @@ const O_CREAT = fs.constants.O_CREAT || 0;
 const O_EXCL = fs.constants.O_EXCL || 0;
 const O_RDONLY = fs.constants.O_RDONLY || 0;
 const O_RDWR = fs.constants.O_RDWR || 0;
+const O_SYNC = fs.constants.O_SYNC || 0;
 const O_TRUNC = fs.constants.O_TRUNC || 0;
 const O_WRONLY = fs.constants.O_WRONLY || 0;
 
@@ -17,6 +18,8 @@ const { stringToFlags } = require('internal/fs');
 
 assert.strictEqual(stringToFlags('r'), O_RDONLY);
 assert.strictEqual(stringToFlags('r+'), O_RDWR);
+assert.strictEqual(stringToFlags('rs+'), O_RDWR | O_SYNC);
+assert.strictEqual(stringToFlags('sr+'), O_RDWR | O_SYNC);
 assert.strictEqual(stringToFlags('w'), O_TRUNC | O_CREAT | O_WRONLY);
 assert.strictEqual(stringToFlags('w+'), O_TRUNC | O_CREAT | O_RDWR);
 assert.strictEqual(stringToFlags('a'), O_APPEND | O_CREAT | O_WRONLY);