first `last_modified` test fails on 32-bit builds

[yomimono]

Discovered on submitting the package to opam for the 0.2.0 release, e.g. https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/afbed8bf46d402f9948cd04b0e7e20be57973c33/variant/extras,x86_32-ocaml-5.3,chamelon-unix.0.2.0,tests .

-   [FAIL]        last modified          0   last modified increases on overwrite.
-   [OK]          last modified          1   last modified of a directory refle...
-   [OK]          last modified          2   last modified only goes 1 level deep.

[yomimono]

I initially thought this was a consistent failure of only the first test, but after getting a usable 32-bit repro environment all the last_modified tests are flaky there, which is less concerning.