There's no guarantee (nor requirement) that the filesystem support pre-epoch
modification dates. If it doesn't, the `test` tests were failing to get the
expected results.
Skip the test if it seems the fs doesn't support pre-epoch timestamps
(determined by pre-epoch mt of `oldest` evaluating to 0 or the unix epoch).
This allows us to test that `test` takes numbers with decimal point even in comma-using locales,
to stop those pesky americans from breaking everything again.
(and yes, we use french to keep myself honest)
This would print the default "Argument is invalid" error string, which
is *true* but not super obvious, because `test` doesn't always perform
numeric conversion, and that's the bit that failed here.
