test: do not write fixture in test-require-symlink

test-require-symlink modifies the fixture directory by adding a symlink.
Copy the fixture to the test tmpdir instead of modifying the fixture
directory.

This also uses a more empirical test for checking for the ability to
make symlinks on Windows.

PR-URL: https://github.com/nodejs/node/pull/15067


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>