[tests] fix notifications test

This was because WS connection wasn't closed properly.
Status Job ID Name Coverage
  Cabal
failed #15548
cabal

 
  Stack
skipped #15549
stack
 
  Bench
skipped #15550
allowed to fail manual
bench
 
  Test
skipped #15551
test
 
Name Stage Failure
failed
cabal Cabal There has been a timeout failure or the job got stuck. Check your timeout limits or try again
No job trace