Bump luatest to 0.5.7-6-gee1f5c1

New version includes the following commits:

- Describe positional arguments in the help message [1]
- Fix doc build warnings associated with luatest [2]
- Add new module tarantool.lua [3]
- Add new func `find_fiber_by_name` to tarantool.lua [4]
- Add new module replica_proxy.lua [5]
- Improve doc strings in tarantool.lua [6]

[1] tarantool/luatest@3c0635c
[2] tarantool/luatest@f639ce4
[3] tarantool/luatest@11b8cfe
[4] tarantool/luatest@427865f
[5] tarantool/luatest@faddd6c
[6] tarantool/luatest@ee1f5c1

Part of tarantool/luatest#238
Part of tarantool/luatest#251