test: increase timeouts in on_shutdown tests

I got four fails on the given tests in a row on debug-asan job in CI for
tarantool-ee.

It seems, tarantool-ee is more sensitive to small timeouts, when the
address sanitizer slows down the execution. Or I'm just lucky.

Anyway, the given tests don't really need small timeouts: increasing it
doesn't break any test logic, doesn't increase duration of the test in a
successful case and doesn't increase it in case of a failure.

The tests are more stable after the change: I verified it locally by
running each of the tests in parallel many times on tarantool built with
enabled address sanitizer.

See the following commits for details about the given test cases and the
problems behind.

* commit 1fcfb8c2 ("app: start init script event loop explicitly")
* commit 786eb2ac ("main: don't break graceful shutdown on init
  script exit")

Follows up #9266
Follows up #9411

NO_DOC=test adjustment
NO_CHANGELOG=see NO_DOC