When searching a topic in Gnus, search all topic's groups

Bug#39515

* lisp/gnus/nnir.el (gnus-group-make-nnir-group): Bring the code in
line with the documentation, which says that all topic groups will be
searched, even if they're not visible.
1 job for master in 63 minutes and 6 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #5008
test-all

01:03:06

 
Name Stage Failure
failed
test-all Test
Makefile:319: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:294: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1