test: don't use cwd for relative path

With the introduction of temporary paths in the test runner
realpath tests would bail in scenarios where the temporary folder
wasn't in the same directory as the source code.

PR-URL: #4477
Reviewed-By: Rich Trott <rtrott@gmail.com>

Author: jbergstroem

test/parallel/test-fs-realpath.js

@@ -236,11 +236,10 @@ function test_relative_input_cwd(callback) {
     return runNextTest();
   }
 
-  // we need to get the relative path to the tmp dir from cwd.
-  // When the test runner is running it, that will be .../node/test
-  // but it's more common to run `./node test/.../`, so detect it here.
+  // we need to calculate the relative path to the tmp dir from cwd
   var entrydir = process.cwd();
-  var entry = common.tmpDir.substr(entrydir.length + 1) + '/cycles/realpath-3a';
+  var entry = path.relative(entrydir,
+      path.join(common.tmpDir + '/cycles/realpath-3a'));
   var expected = common.tmpDir + '/cycles/root.js';
   [
     [entry, '../cycles/realpath-3b'],