tests/dcerpc/misc.GUID: improve tests

1. Merge tests for different formats into a for loop, make it easy to
read and extend.
2. Add test for invalid formats.

Signed-off-by: Joe Guo <joeg@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>