mon: create simple should goto reply when it exists in pending

What started with a one liner ( https://github.com/dachary/ceph/commit/ff195d14909c5d3067d7561bcd2e23f232e320cf ) turned out to be the opportunity to fix two similar bugs ( https://github.com/dachary/ceph/commit/f0be218ae0befd747fd0ac0ffe427390aac5c0dc and https://github.com/dachary/ceph/commit/1123d5997702d4c696582abcbfbb550961f08a35 ), factorize the mon functional tests run on **make check** and find a reasonably simple way to reach the modified code although it only happens on a border case.