Merging platforms mixes baselines and targets into reported data

Merging platforms mixes baselines and targets into reported data
https://bugs.webkit.org/show_bug.cgi?id=135260

Reviewed by Andreas Kling.

When merging two platforms, move test configurations of a different type (baseline, target)
as well as of different metric (Time, Runs).

Also avoid fetching the entire table of runs just to see if there are no remaining runs.
It's sufficient to detect one such test_runs object.

* public/admin/platforms.php:
(merge_platforms):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@172410 268f45cc-cd09-0410-ab3c-d52691b4dbfc