Shutdown of mtr tests may be too impatient, esp on CI environment where
10 seconds of `arg` of `shutdown_server arg` may not be enough for the clean
shutdown to complete.

This is fixed to remove explicit non-zero timeout argument to
`shutdown_server` from all mtr tests. mysqltest computes 60 seconds default
value for the timeout for the argless `shutdown_server` command.
This policy is additionally ensured with a compile time assert.