clickhouse: add more tests for TFW Logger options

[RomanBelozerov]

Now we have one tests for that - t_clickhouse.test_clickhouse_logs.TestClickhouseLogsBufferConfiguration.test_mmap_buffer. It checks that TempestaFW starts with non-default mmap_log_buffer_size. Please add more tests for this directive and other clickhouse options

[symstu-tempesta]

Move t_stress/t_clickhouse tests into t_clickhouse and run them with light traffic to verify everything works correctly. Also, add a check for buffer overload under stress by verifying dropped requests in the access_log table.