Don't give up if no results arrive after 500ms

Summary:
Currently it resets the entire context if no results appear after 500ms,
which means no new results can appear. This makes no sense.

RunnerManager does not emit any matchChanged signal while the runners update
the matches too quickly. The way this delay is implemented can result in never
actually emitting the signal until all runners are done though.
Thus it's actually likely to hit this situation.

BUG: 389611

Test Plan:
Test results appear now, even if the first batch of runners is slow.
List of matches still clears if nothing was found.

Reviewers: Plasma

Subscribers: broulik, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D21605