translateBlockExpiry: Duration is block expiry minus current time

translateBlockExpiry: Duration is block expiry minus current time

For relative timestamps in $str, strtotime( $str, $now ) returns an
absolute Unix timestamp $str since $now, and this timestamp is given
to $time. However, Language::formatDuration expects a time duration,
not an absolute timestamp. We obtain this duration from the difference
between $time, the absolute timestamp of block expiry, and $now, the
absolute timestamp of the time in which the block action happened.

Tests have been added to test both this patch and 01936fa, the patch
that caused this regression.

Bug: T156453
Change-Id: I6fd8c02dc3c6456067fe25cb9f33f5b4c78332aa