tests: Move test_dom_suite_setup to different module
    
    The function test_dom_suite_setup is a simple function which creates
    a directory which should be used for creating files. This function
    needn't be linked with libsss_util.so as other functions in common_dom.c
    This is a reason why it's better to move it to the file common.c
    so linker can optimize out other unnecessary module objects from static
    library libsss_test_common.a and test needn't be linked libsss_util.so.
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>