test: refactor test-child-process-double-pipe

Update test to use strictEqual instead of equal to remove error.

PR-URL: #9930
Reviewed-By: Prince John Wesley <princejohnwesley@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-child-process-double-pipe.js

@@ -91,5 +91,5 @@ sed.stdout.on('data', function(data) {
 });
 
 sed.stdout.on('end', function(code) {
-  assert.equal(result, 'hellO' + os.EOL + 'nOde' + os.EOL + 'wOrld' + os.EOL);
+  assert.strictEqual(result, `hellO${os.EOL}nOde${os.EOL}wOrld${os.EOL}`);
 });