update_engine: fix lint issues in DeltaPerformer tests

Two lint issues:
* nondeterminism caused by rand(), replaced with rand_r()
  [runtime/threadsafe_fn]
* two spaces between code and comments [whitespace/comments]

The latter was resolved by moving out the comment and what it was
attached to.

BUG=None
TEST=unit tests

Change-Id: Ie29237c578e34c9f5f22fca984c5e9bf5a54b1a7
Reviewed-on: https://chromium-review.googlesource.com/882559


Commit-Ready: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
Reviewed-by: Amin Hassani <ahassani@chromium.org>