Make test suite faster

[fingolfin]

We achieve this in multiple ways:

• for groups of order <= 100 use RegularActionHomomorphism instead of IsomorphismPermGroup, as it is much faster there and we don't care about the larger degree of the resulting permutation representations
• when we have to copies of the same group (sot[i] and soty[i]) don't bother to use getRandomPc on both if they use identical presentations
• rewrite getRandomPc to use a better algorithm that avoids computing all maximal subgroups (expensive) when all we want are normal subgroups of prime index

Before:

125754 ms (19144 ms GC) and 44.8GB allocated

After:

56931 ms (6719 ms GC) and 17.4GB allocated

[codecov[bot]]

Codecov Report

Merging #52 (366d59a) into master (3d4489f) will decrease coverage by 0.03%. The diff coverage is 96.87%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
- Coverage   98.35%   98.33%   -0.03%     
==========================================
  Files          21       21              
  Lines        7727     7747      +20     
==========================================
+ Hits         7600     7618      +18     
- Misses        127      129       +2     

Files Changed	Coverage Δ
tst/test.gi	87.67% <96.87%> (+1.95%)	:arrow_up:

... and 1 file with indirect coverage changes