Run coverage tests with --concurrency=1 too

Due to various race conditions we've been running unit tests with
--concurrency=1 for a long time now. The same must be done to coverage
tests too.

Change-Id: Ie402ee005043e250ee5778a00efc16db588ed811
Closes-Bug: #1252743
Merge "Run coverage tests with --concurrency=1 too"