[PendingFileQueue] Use more accurate calculation of remaining time

Summary:
QTime::secsTo completely ignores any fractional (sub-second) part, i.e.
1.001 to 2.999 is a difference of 1 second, as is 1.999 to 2.001.

This leads to bursts when processing the pending queue, and also makes
testing more difficult, as the timeouts trigger with a jitter of
+/- 1 second.

Test Plan: ctest

Reviewers: Baloo, ngraham, astippich, poboiko

Reviewed By: Baloo, ngraham

Subscribers: kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D21790