test: make a test path-independent

parallel/test-spawn-cmd-named-pipe.js failed with spaces
both in node.exe and test paths.

PR-URL: nodejs#12945
Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

test/parallel/test-spawn-cmd-named-pipe.js

@@ -37,15 +37,10 @@ if (!process.argv[2]) {
   });
   stdoutPipeServer.listen(stdoutPipeName);
 
-  const comspec = process.env['comspec'];
-  if (!comspec || comspec.length === 0) {
-    assert.fail('Failed to get COMSPEC');
-  }
+  const args =
+    [`"${__filename}"`, 'child', '<', stdinPipeName, '>', stdoutPipeName];
 
-  const args = ['/c', process.execPath, __filename, 'child',
-                '<', stdinPipeName, '>', stdoutPipeName];
-
-  const child = spawn(comspec, args);
+  const child = spawn(`"${process.execPath}"`, args, { shell: true });
 
   child.on('exit', common.mustCall(function(exitCode) {
     stdinPipeServer.close();