Update writetimeout to be T_IDLE in netmgr_test.c

Update writetimeout to be T_IDLE in netmgr_test.c

Use the isc_nmhandle_setwritetimeout() function in the netmgr unit test
to allow more time for writing and reading the responses because some of
the intervals that are used in the unit tests are really small leaving a
little room for any delays.

(cherry picked from commit ee359d6ffa701e5f9781ec75622af22736506bdd)