Fix one IsPNilpotent method, and add some tests

[fingolfin]

Fixes #2099

[codecov]

Codecov Report

Merging #2106 into master will decrease coverage by 12.07%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master    #2106       +/-   ##
===========================================
- Coverage   69.22%   57.15%   -12.08%     
===========================================
  Files         489      426       -63     
  Lines      256253   221359    -34894     
===========================================
- Hits       177395   126508    -50887     
- Misses      78858    94851    +15993

Impacted Files	Coverage Δ
lib/grp.gi	71.9% <100%> (-12.56%)	⬇️
lib/attr.gi	0% <0%> (-100%)	⬇️
lib/teachm2.g	0% <0%> (-100%)	⬇️
lib/contfrac.gi	0% <0%> (-89.48%)	⬇️
grp/imf.gi	0% <0%> (-84.96%)	⬇️
lib/proto.gi	0% <0%> (-83.34%)	⬇️
lib/ctblauto.gi	0% <0%> (-82.41%)	⬇️
lib/ctbllatt.gi	0% <0%> (-80.72%)	⬇️
lib/teaching.g	2.45% <0%> (-77.15%)	⬇️
lib/algliess.gi	0% <0%> (-75.23%)	⬇️
... and 367 more

[olexandr-konovalov]

thanks @fingolfin