[observer|test] Ensure observer stopped
[observer|test] Ensure observer stopped

Stopping observer is actually asynch, just calling stop is no
guarantee that it actually stopped.
On some (test) platforms, test cases failed because they could
not start observer; because it was already running.

OTP-17346

ferd/otp,erlang/otp,getong/otp,bjorng/otp,g-andrade/otp,jj1bdx/otp,emacsmirror/erlang,isvilen/otp,uabboli/otp,bjorng/otp,emacsmirror/erlang,dumbbell/otp,isvilen/otp,getong/otp,jj1bdx/otp,g-andrade/otp,emacsmirror/erlang,potatosalad/otp,bjorng/otp,bjorng/otp,dumbbell/otp,ferd/otp,getong/otp,rlipscombe/otp,vinoski/otp,mikpe/otp,emacsmirror/erlang,getong/otp,dumbbell/otp,emacsmirror/erlang,dumbbell/otp,dgud/otp,getong/otp,vinoski/otp,dgud/otp,dumbbell/otp,isvilen/otp,rlipscombe/otp,g-andrade/otp,jj1bdx/otp,potatosalad/otp,mikpe/otp,bjorng/otp,erlang/otp,g-andrade/otp,mikpe/otp,jj1bdx/otp,erlang/otp,vinoski/otp,ferd/otp,erlang/otp,rlipscombe/otp,vinoski/otp,bjorng/otp,jj1bdx/otp,dgud/otp,jj1bdx/otp,dgud/otp,vinoski/otp,dumbbell/otp,isvilen/otp,erlang/otp,uabboli/otp,isvilen/otp,dgud/otp,vinoski/otp,dumbbell/otp,jj1bdx/otp,jj1bdx/otp,isvilen/otp,uabboli/otp,dumbbell/otp,g-andrade/otp,erlang/otp,vinoski/otp,mikpe/otp,getong/otp,mikpe/otp,rlipscombe/otp,erlang/otp,vinoski/otp,mikpe/otp,getong/otp,g-andrade/otp,dgud/otp,ferd/otp,jj1bdx/otp,erlang/otp,dgud/otp,uabboli/otp,mikpe/otp,ferd/otp,ferd/otp,mikpe/otp,isvilen/otp,emacsmirror/erlang,emacsmirror/erlang,bjorng/otp,potatosalad/otp,dumbbell/otp,mikpe/otp,isvilen/otp,ferd/otp,rlipscombe/otp,rlipscombe/otp,mikpe/otp,getong/otp,bjorng/otp,potatosalad/otp,dgud/otp,dumbbell/otp,potatosalad/otp,getong/otp,uabboli/otp,jj1bdx/otp,rlipscombe/otp,uabboli/otp,isvilen/otp,vinoski/otp,getong/otp,dgud/otp,potatosalad/otp,rlipscombe/otp,uabboli/otp,g-andrade/otp,uabboli/otp,erlang/otp,ferd/otp,potatosalad/otp,g-andrade/otp,bjorng/otp,rlipscombe/otp,potatosalad/otp,emacsmirror/erlang,g-andrade/otp,potatosalad/otp,vinoski/otp,ferd/otp,g-andrade/otp,bjorng/otp,isvilen/otp,uabboli/otp,emacsmirror/erlang,rlipscombe/otp,dgud/otp,potatosalad/otp,emacsmirror/erlang,erlang/otp