This adds regression tests for fromTOML overflow/underflow behavior.
Previous versions of toml11 used to saturate, but this was never an
intended behavior (and Snix/Nix 2.3/toml11 >= 4.0 validate this).
(cherry picked from Lix [1,2])
[1]: 7ee442079d
[2]: 4de09b6b54
