sens: add check_sample_rate function to verify if events frequency exceeds some rate...

sens: add check_sample_rate function to verify if events frequency exceeds some rate per second

Allows passing the rate as parameter to the check_sample_rate method.
Starts polling for one second or until a number of events, equal with the
rate, is reached. Finally, verifies if the rate of events per second is
reached (if it is true, will print "test passed", otherwise will print
"test failed")
ex: sens check_sample_rate 100
Will print "test passed" if the number of events in 1 second exceeds 100,
otherwise will print "test failed".

Change-Id: Ic84e9fe13a577435bf55304b0aecafdd83b00f47
Signed-off-by: Maria Constantin <maria.p.constantin@intel.com>