test: refactor dbgr test with `it` helper

`console_debugger_session_test.lua` can be simplified and made more
stable with interactive_tarantool helper. With the new
`read_untill_prompt()` helper function, dbgr prompt can be used as
end-of-output marker instead of '...' in yaml-outputting main console.
This way, popen results are unambiguous and no retries are required.
At the same time, dbgr prompt is now expected with every result, thus
there is no need for an extra check. But `continue` command usually
reaches the end of the script and exits debugger, thus
'<END_OF_EXECUTION>' marker was introduced.
Empty stderr and header check were moved to interactive_tarantool helper.
This patch breaks the test without debugger readline support patch,
that is next in the patchset.

Part of #7738

NO_CHANGELOG=test refactoring
NO_DOC=test refactoring

(cherry picked from commit 72008443)