conftests: allow simultaneous 'wait_for', improve databaseds process synchronisation

Had to add a short sleep, to sync with databaseds process start ;
could be improved by digging more into PyTango, but it can be enough like
this as well...
4 jobs for 651-serial-refactoring in 39 minutes and 48 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Style
passed #38630
check_style

00:00:18

 
  Tests
passed #38631
run_tests:source

00:34:27

33.0%
 
  Build
passed #38632
create_doc:reference

00:05:02

passed #38633
create_doc:user

00:01:40