test/zdtm/sockets_dgram: check the length of path used

Path generated in the test is copied into sun_path attribute in
struct sockaddr_un. Additional check on whether sun_path can
contain the whole path is added. It is useful because if sun_path
can only contain the first part of the path, all sockadd_un
will point to the same location, which leads to error.

Signed-off-by: Yicheng Qin <yichengq@google.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>