tests: Add some delay before wlantest operations

tests: Add some delay before wlantest operations

Since wlantest is a separate process that is not synchronized with rest
of the test components, there can be some latency in it having the STA
entries and counters updated. There is a race condition between this
happening and then test script clearing or fetching data. Make this race
condition less likely to cause bogus test failures by adding some wait
between these operations.

Signed-hostap: Jouni Malinen <j@w1.fi>