tests: Setup wlantest once for qosmap tests

tests: Setup wlantest once for qosmap tests

Some tests call the check_qos_map() function more than once. Make sure
each test sets up wlantest only once before the first time the function
is called.

The wlantest setup sets the channel for the wlantest interface and
executes the wlantest executable. It is more efficient to do that only
once for each test.

Signed-off-by: Jonathan Afek <jonathanx.afek@intel.com>