Add more tests

typechecker/src/Tc/SkolSub.hs

typechecker/src/Tc/Solver/Equality.hs

typechecker/src/Tc/Solver/Monad.hs

typechecker/test/cases/should-typecheck/bottom-infer.hs

typechecker/test/cases/should-typecheck/bottom-infer.output

typechecker/test/cases/should-typecheck/fix-infer.hs

typechecker/test/cases/should-typecheck/fix-infer.output

typechecker/test/cases/should-typecheck/fixLet-infer.hs

typechecker/test/cases/should-typecheck/fixLet-infer.output