Add some tests for permutations

[fingolfin]

This PR begins adding additional tests for permutations, working systematically through src/permutat.c.

We might want to add similar tests for partial permutations and transformations, but I know little about them.

[codecov]

Codecov Report

Merging #1145 into master will increase coverage by 0.37%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1145      +/-   ##
==========================================
+ Coverage   59.58%   59.95%   +0.37%     
==========================================
  Files         934      933       -1     
  Lines      292154   290265    -1889     
  Branches    18148    17611     -537     
==========================================
- Hits       174072   174032      -40     
+ Misses     113713   111868    -1845     
+ Partials     4369     4365       -4

Impacted Files	Coverage Δ
src/permutat.c	53.31% <100%> (+2.26%)	⬆️
hpcgap/src/funcs.c	48.74% <0%> (-0.53%)	⬇️
hpcgap/src/vars.c	37.84% <0%> (-0.18%)	⬇️
src/scanner.c	70.16% <0%> (+0.08%)	⬆️
src/hpc/threadapi.c	28.21% <0%> (+0.25%)	⬆️
src/hpc/tls.h	67.5% <0%> (+2.5%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bcb0002...279023c. Read the comment docs.

[markuspf]

fwiw trans.c has coverage of 99+%, its one of the model citizens of src/..

[fingolfin]

Excellent -- I didn't check before writing that remark (I actually only thought of it when writing th PR description, should have checked). Anyway, I actually see trans.c at "only" 95-96% (I am not complaining; there are always some bits that are tough to reach). Also pperm.c has a respectable 85-86 while for permutat.c it's only 44-55%. (I give number ranges, because I see something different on master vs. https://codecov.io/gh/gap-system/gap/tree/7de6e7274e9a91fc80dd5f35ede176be181c32d9/src but I hope this will stabilize once PR #1143 is ready and merged)

[markuspf]

#1130 takes it to literally 99% though (and I only know this because we had the discussion about test coverage today ;))