fix _as_subgroups

[ThomasBreuer]

and extend tests accordingly,
and fix == for group homomorphisms

resolves #4276

[joschmitt]

Thank you!

The failing book test seems to be vinberg2.jlcon again, I restarted.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.49%. Comparing base (50d2662) to head (21c4ab2).
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4277      +/-   ##
==========================================
- Coverage   84.50%   84.49%   -0.01%     
==========================================
  Files         641      641              
  Lines       85473    85467       -6     
==========================================
- Hits        72225    72217       -8     
- Misses      13248    13250       +2     

Files with missing lines	Coverage Δ
src/Groups/homomorphisms.jl	91.17% <100.00%> (ø)
src/Groups/sub.jl	95.65% <100.00%> (-0.14%)	⬇️

... and 1 file with indirect coverage changes