app: start init script event loop explicitly

The motivation is to reduce time slip on Tarantool startup before
running init scripts. Internal ev time is set in fiber_init/ev_default_loop
and is not get updated until starting event loop. This causes
timeouts slip up to 0.3 in debug ASAN build in init script (see #9261).

Let's run event loop right at the beginning of the run_script_f before
executing any script. This way besides updating internal ev time we make
an explicit place of starting script event loop. Currently it is started
lazily when config script yields.

This will fix CI for PR https://github.com/tarantool/tarantool-ee/pull/572
for debug ASAN workflow.

We can also remove start_loop condition. It does not make sense now. It
was added in the commit 3a851430 ("Fix tarantool -e "os.exit()"
hang") but since then we start to stop event loop after handling
os.exit().

Also this fixes #9266. The issue is we don't have an event loop to run
on shutdown triggers if -e command line expression add such a trigger
and then call os.exit().

Follow-up #7327
Closes #9266

NO_DOC=bugfix

(cherry picked from commit 1fcfb8c2)