run-in-tempdir: Improve speed
run-in-tempdir: Improve speed

By creating symlinks instead of actually creating files we still get the
benefits of using a temporary directory, but the preparation is about
three times as fast. Test used:

$ time bash -c 'for ((i=0; i < 100; ++i)); do
    autotools/run-in-tempdir true; done'

Signed-off-by: Michael Hanselmann <68b67885ff74281219b4a4be33cc20fd45078d7c@google.com>
Reviewed-by: Bernardo Dal Seno <dcdd79c5479cb9d1329ec01d126f02cb045aa103@google.com>

grnet/snf-ganeti,yiannist/ganeti,leshchevds/ganeti,yiannist/ganeti,apyrgio/ganeti,yiannist/ganeti,mbakke/ganeti,andir/ganeti,bitemyapp/ganeti,grnet/snf-ganeti,leshchevds/ganeti,andir/ganeti,ganeti/ganeti,bitemyapp/ganeti,apyrgio/snf-ganeti,leshchevds/ganeti,dimara/ganeti,onponomarev/ganeti,apyrgio/ganeti,ganeti-github-testing/ganeti-test-1,dimara/ganeti,mbakke/ganeti,ganeti/ganeti,ganeti/ganeti,mbakke/ganeti,onponomarev/ganeti,ganeti-github-testing/ganeti-test-1,apyrgio/snf-ganeti,andir/ganeti