libraVNA.cmd() no longer expects blank line responses. By default,
it checks status after each command to match the previous behavior.
Check can be disabled by optional cmd() parameter for handling expected
failure cases.
Commands such as *WAI may take longer than anything previously. Make
it possible to override the default time-out in cmd() and query().
Change poll loops in TestCalibration and TestVNASweep to *WAI calls.