Fix warnings issued by "forge lint"

[PaulRBerg]

e.g. https://getfoundry.sh/forge/linting/#mixed-case-function

note[mixed-case-function]: function names should use mixedCase
  --> /Users/prb/Projects/prb-math/test/unit/ud60x18/math/mul/mul.t.sol:46:14
   |
46 |     function mul_Sets() internal returns (Set[] memory) {
   |              --------
   |
   = help: https://book.getfoundry.sh/reference/forge/forge-lint#mixed-case-function

note[mixed-case-function]: function names should use mixedCase
   --> /Users/prb/Projects/prb-math/test/unit/sd59x18/math/mul/mul.t.sol:149:14
    |
149 |     function operandsDifferentSigns_Sets() internal returns (Set[] memory) {
    |              ---------------------------
    |
    = help: https://book.getfoundry.sh/reference/forge/forge-lint#mixed-case-function