A new root logger is installed when a crawler is created
if one was already installed before.
This allows to respect custom settings related to logging,
such as LOG_LEVEL, LOG_FILE, etc.
* use CrawlerRunner.create_crawler instead of get_crawler helper in test_crawl;
* add a test for loading spiders by name;
* add a test for passing Crawler objects instead of Spider objects;
* add a test for CrawlerRunner.join
This allows to remove `get_testlog` helper, `flushLoggedErrors` from
twisted.trial.unittest.TestCase and Twisted log observers created for
each test on conftest.py.