test: correct app-luatest/trigger_module_test.lua

Module trigger should be cleaned up after each test. It didn't cause any
troubles because each test used a unique name of event for its purposes.
Let's remove all registered triggers after each test for the sake of
maintainability.

Also, move the tests to a separate server instance - currently tests use
tarantool which is a test runner, so it may affect other tests during a
competitive launch.

NO_CHANGELOG=test
NO_DOC=test