test: remove unnecessary default tmpdir value in test

`tmpdir.path` will never be falsy so there is no need to guard against
that.

PR-URL: nodejs#26177
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Weijia Wang <starkwang@126.com>

test/parallel/test-pipe-file-to-http.js

@@ -29,7 +29,7 @@ const path = require('path');
 const tmpdir = require('../common/tmpdir');
 tmpdir.refresh();
 
-const filename = path.join(tmpdir.path || '/tmp', 'big');
+const filename = path.join(tmpdir.path, 'big');
 let count = 0;
 
 const server = http.createServer((req, res) => {