Update developer-guide based on improving Xtesting

This patch updates the developer-guide according to the change in
Xtesting improvement [1].

[1] https://gerrit.o-ran-sc.org/r/c/smo/o2/+/10808

Issue-ID: SMO-131
Signed-off-by: Koichi Edagawa <edagawa.kc@nec.com>
Change-Id: Id6f97f81cdd46096d1850f94c4ddd54eb55ce5c6
1 file changed