selftest: Only set clockskew to 5 seconds for MIT Kerberos

selftest: Only set clockskew to 5 seconds for MIT Kerberos

This was added in ac5427c6eba09134411f76a5e6f7e2643fa74eed as part of the MIT KDC
effort, but makes some tests much less reliable under high load.

As the Heimdal build does not need this, only specify for the MIT build.

Tested with an MIT AD DC build with:
 make test TESTS="samba3.raw.session samba3.smb2.session"

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>
Autobuild-User(master): Andrew Bartlett <abartlet@samba.org>
Autobuild-Date(master): Wed Feb 13 05:49:43 CET 2019 on sn-devel-144