twister: fix build dir path for mklink

When twister is run on Windows with --short-build-path option, mklink
fails to create link, because path to build dir contains forward
slashes, which are not handled correctly by mklink.
Fix it by using os.path.normpath in mklink call.
Added os.path.join mock in twister unit test to handle path join
consistently.

Signed-off-by: Michal Smola <michal.smola@nxp.com>