Make reuse_shared_userspace_init_data faster
* A significant performance improvement is to avoid repeating call of
mmd.get_xmd() in a looping.
* Function is refactored to avoid call of mmd_to_str twice for a module
build.
* Test using this fixture already has a setup_method to clean database,
so no need to call clean_database again.
* Calculate build context directly without calculating other contexts
together.
Signed-off-by: Chenxiong Qi <cqi@redhat.com>