tests: fix test coverage reporting for parallel testing.

* Fix simplecov clobbering test coverage reports when using parallel tests.
* Generate coverage reports by default (remove $SIMPLECOV flag).
* Store coverage reports in tmp/coverage/ instead of coverage/.
* Enable branch coverage.

ref: github.com /colszowka/simplecov/issues/718#issuecomment-538201587

.gitignore

@@ -7,7 +7,6 @@ config/database.yml
 config/danbooru_local_config.rb
 config/deploy/*.rb
 config/newrelic.yml
-coverage/
 node_modules/
 log/
 tmp/