Fix relative format showing now for any past time under a minute - #390

Fixes #269

#getRelativeFormat in src/relative-time-element.ts turns anything under 10 seconds into "now":

if (unit === 'second' && int < 10) {

For past dates int is negative, so this is always true and every past time under a minute renders as "now". Future dates are fine, which is why the table tests expect "in 50 seconds" for 50 seconds ahead but "now" for 50 seconds ago.

<!-- 30 seconds ago -->
<relative-time datetime="..." format="relative"></relative-time>
<!-- before: now, after: 30 seconds ago -->

Before #getRelativeFormat switched to durations, timeAgo returned "now" under 10 seconds and "N seconds ago" from 10 to 44 seconds, so this looks like a regression from that change. The fix compares Math.abs(int).

I updated the two "50 seconds in the past" table tests that encoded the old output. They fail before the change and pass after. I ran the full test suite (web-test-runner, Chromium), tsc --noEmit and eslint.