Replace use of ASSERT in test code.

In top level test functions, replaced with gtest ASSERT_*. In helper
methods in main test files, replaced with EXPECT_* or RTC_DCHECK on a
case-by-case basis.

In separate mock/fake classes used by tests (which might be of some
use also in tests of third-party applications), ASSERT was replaced
with RTC_CHECK, using

  git grep -l ' ASSERT(' | grep -v common.h | \
    xargs sed -i 's/ ASSERT(/ RTC_CHECK(/'

followed by additional includes of base/checks.h in affected files,
and git cl format.

BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2622413005
Cr-Commit-Position: refs/heads/master@{#16150}