test: allow to execute custom code before box.cfg in default instance

Sometimes there's a need in creating a custom testing instance, with
test-specific code, which doesn't fit in box.cfg.

Our usual approach is to duplicate the instance file (default.lua) and make
the necessary changes there. This leads to a lot of code duplication and
lots of instance files with unclear use cases.

Let's better make it possible to pass code to the default instance via
an env variable TARANTOOL_RUN_BEFORE_CFG.

This will be used in box.ctl.on_recovery_state() triggers test.

In-scope-of #5272

NO_DOC=testing
NO_TEST=testing
NO_CHANGELOG=testing