tests: simulate old LastScan wifi value in test-networkmanager-service.py

In this way clients will randomly find that the AP list is older than
30 seconds and they will issue a new scan.
4 jobs for bg/issue39 in 19 minutes and 51 seconds (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Test
passed #39067
allowed to fail
checkpatch

00:02:25

passed #39068
f28_build_default

00:10:42

passed #39069
f28_rpm_autotools_dbg

00:10:56

passed #39070
f28_rpm_meson_dbg

00:09:36