Now it is possible to mark test files with tags and run some group of
tests using the `--tags` option.

Usage:

 | ./test-run.py --tags foo
 | ./test-run.py --tags foo,bar app/ app-tap/

Show the list of tags:

 | ./test-run.py --tags
 | ./test-run.py app-tap/ --tags

See syntax and more details in
https://github.com/tarantool/test-run/pull/314

See the task description and possible usages in
https://github.com/tarantool/test-run/issues/22