test: use luatest modules instead of internal ones

Some internal modules have been recently copied to luatest repo [1,2,3]
and now they can be safely removed, and the corresponding functionality
from luatest can be used instead.

Affected modules:

- test/luatest_helpers/misc.lua
- test/luatest_helpers/fiber.lua
- test/luatest_helpers/proxy/*

[1] https://github.com/tarantool/luatest/pull/247
[2] https://github.com/tarantool/luatest/pull/248
[3] https://github.com/tarantool/luatest/pull/255

Closes tarantool/luatest#238
Closes tarantool/luatest#251

NO_DOC=testing stuff
NO_TEST=testing stuff
NO_CHANGELOG=testing stuff