tail: fix big number handling and error message quoting

[ChrisDryden]

When looking at the test cases in tail.pl I concerned that I could find equivalent examples in our integration tests that expect different results. It turns out that this is because the behavior of tail actually changed in 9.6 to clamp large integers instead of throwing an error, so some of our code was no longer valid.

Because we already have the helper functions that follow this behavior the change is very simple, changing parse_signed_num to parse_signed_num_max and then changing:

 let Some(num) = num.checked_mul(multiplier) else {
        return Some(Err(ParseError::Overflow));
    };

to

    let num = num.saturating_mul(multiplier);

Then there was an error message that had an extra quote and now all of the tail.pl test cases are passing

[github-actions]

GNU testsuite comparison:

Congrats! The gnu test tests/tail/tail is no longer failing!

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/tail is no longer failing!