Store files created by tests in separate directories

[locker]

Some luatest helpers, for example luatest.server, create temporary files right in the test run directory (/tmp/t/ by default). It may be confusing when more than one test file is executed or more than one instance of luatest is running (as it's the case with using ctest -j in Tarantool). It'd be nice if luatest created a sub-directory for all temporary files and logs per each executed test file. All these files should be saved as artifacts on test failure.