Prefer `expect` over `assert` for testing

Replaces `assert` with `expect` in tests.  This commit covers all tests
in acceptance and helper folders as specified on #2797.
Merge remote-tracking branch 'upstream/master'
Fix `expect` jshint errors.

Fixes expect jshint errors added via #2842.
Removed `expect` chains

Cleaned up `expect` chains and extracted empty directory check to a
separate file.
Merge pull request #2842 from stavarotti/master

Prefer `expect` over `assert` for testing