[garnet] wlan-stress-test: Added support for scan

Added support to perform stress test of the wlan scan API if -s is
passed as an option. The test will repeat calls to scan based on the -r
option.

Bug: WLAN-648
Test: build and boot garnet
Test: run wlan-stress-test -s -r 2 and confirm that scan is called two
      times.
Test: run wlan-stress-test -s -c -d -r 2 GoogleGuest and confirm scan,
      connect and disconnect are called two times and a report is generated.

Change-Id: I2ebb007ee5e38b89e01c4010a71b6df14542f537
1 file changed