path: fixing a test that breaks on some machines.

A win32-only test was verifying that path.win32._makeLong('C:')
would return the current working directory.  This would only work if
current working directory was also on the C: device.  Fix is to grab
the device letter for current working directory, and pass that to
_makeLong().

PR-URL: #6067
Reviewed-By: Trott - Rich Trott <rtrott@gmail.com>
Reviewed-By: Joao Reis <reis@janeasystems.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>

test/parallel/test-path.js

@@ -562,7 +562,8 @@ if (common.isWindows) {
                '\\\\?\\' + process.cwd().toLowerCase() + '\\foo\\bar');
   assert.equal(path.win32._makeLong('foo/bar').toLowerCase(),
                '\\\\?\\' + process.cwd().toLowerCase() + '\\foo\\bar');
-  assert.equal(path.win32._makeLong('C:').toLowerCase(),
+  const currentDeviceLetter = path.parse(process.cwd()).root.substring(0, 2);
+  assert.equal(path.win32._makeLong(currentDeviceLetter).toLowerCase(),
                '\\\\?\\' + process.cwd().toLowerCase());
   assert.equal(path.win32._makeLong('C').toLowerCase(),
                '\\\\?\\' + process.cwd().toLowerCase() + '\\c');