Simplify testRandGmres

[shakedregev]

Description

The helper function for the test generateMatrix was overly convoluted. See #223.

Closes #223.

Proposed changes

Simplified conditionals, logic, added more expressive variable names, and documentation.

Checklist

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• All tests pass. Code tested on
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  • CUDA backend
  • HIP backend
• Code compiles cleanly with flags -Wall -Wpedantic -Wconversion -Wextra.
• The new code follows Re::Solve style guidelines.
• There are unit tests for the new code.
• The new code is documented.
• The feature branch is rebased with respect to the target branch.
• Included within changelog scope.

[nkoukpaizan]

Changes make sense to me, and tests pass on Frontier and pdesx. This is a lot cleaner!

[pelesh]

Thanks for cleaning this up!