Bump luatest to 0.5.7-33-g8523e5c

    New version includes the following commits related to luatest
    functionality:

    - Add Tarantool 2.10 to tests workflow [1]
    - Not fail server:drop() if the process is already dead [2]
    - Remove server:clean() method [3]
    - Save server artifacts if the test fails [4]

    [1] tarantool/luatest@4475cfd
    [2] tarantool/luatest@abe5d42
    [3] tarantool/luatest@847e7b7
    [4] tarantool/luatest@8523e5c