Remove luatest submodule

[locker]

Having luatest as a submodule is inconvenient because:

1. To update luatest in some project using it via test-run, we have to update it in test-run first.
2. From point 1 it follows that all projects using test-run are bound to use the same version of luatest.

Let's drop the luatest submodule from test-run and require projects using test-run install luatest as a prerequisite, either as a submodule or from luarocks.