Patched _solve_triu and _solve_tril; beefed up the tests

[JohnAAbbott]

The patches change just 2 lines, but I renamed the args to _solve_tril! so that they correspond to the "doc" in the comment right before the function (and also coherent with _solve_triu). The tests are now with 3x3 matrices since the old buggy code happened to work with 2x2 matrices.

[JohnAAbbott]

I note that currently the documentation for _solve_triu is in AbstractAlgebra; in particular near line 3498 in the file src/Matrix.jl there is the following:

# Remove the following once Nemo is adjusted
_solve_triu_right(M, b; unipotent = false) = _solve_triu(M, b; unipotent, side = :right)

There is apparently no documentation for _solve_tril!

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.60%. Comparing base (222f2e1) to head (95f986c).
⚠️ Report is 75 commits behind head on master.

Files with missing lines	Patch %	Lines
src/flint/fmpz_mat.jl	86.36%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2177      +/-   ##
==========================================
+ Coverage   88.17%   89.60%   +1.43%     
==========================================
  Files          99      100       +1     
  Lines       37181    48324   +11143     
==========================================
+ Hits        32786    43303   +10517     
- Misses       4395     5021     +626     

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[JohnAAbbott]

Not sure why this was still a draft -- now changed to standard PR.

[fingolfin]

Maybe @JohnAAbbott and @fieker can look into this in the afternoon to get it over the finish line. Also PR #2152