test: replace process.platform check with common.isWindows

Author: injunchoi98

test/parallel/test-child-process-cwd.js

@@ -54,7 +54,7 @@ function testCwd(options, expectPidType, expectCode = 0, expectData) {
   child.on('close', common.mustCall(function() {
     if (expectData) {
       // In Windows, compare without considering case
-      if (process.platform === 'win32') {
+      if (common.isWindows) {
         assert.strictEqual(data.trim().toLowerCase(), expectData.toLowerCase());
       } else {
         assert.strictEqual(data.trim(), expectData);