fix winbuilder problem with file.symlink in test

* not exactly sure why, but there was a permissions error with symlinking
* just copy instead

In file.symlink(normalizePath(amfile), amfilenoam) :
  cannot symlink 'd:\RCompile\CRANguest\R-release\nat.Rcheck\tests_i386\testthat\testdata\amira\AL-a_M.am' to 'D:\temp\Rtmp0MfyyQ\file2e3c18211a5e', reason 'Dem Client fehlt ein erforderliches Recht'
Execution halted