mavsdk tests: use tester sleep_for function

the previously used std::this_thread::sleep_for is with respect to host
system time which is different from autopilot time if:

 - speed factor != 1
 - something runs slower than realtime regardless of speed factor
 - debugging or otherwise interrupting PX4 control code

tester.sleep_for (which already existed) correctly sleeps w.r.t.
px4/simulation time.

test/mavsdk_tests/test_vtol_loiter_airspeed_failure_blockage.cpp

@@ -56,12 +56,12 @@ TEST_CASE("Fly VTOL Loiter with airspeed failure", "[vtol_airspeed_fail]")
 
 
 	// tester.wait_until_altitude(50.f, std::chrono::seconds(30));
-	std::this_thread::sleep_for(std::chrono::seconds(10));
+	tester.sleep_for(std::chrono::seconds(10));
 	tester.inject_failure(mavsdk::Failure::FailureUnit::SensorAirspeed, mavsdk::Failure::FailureType::Wrong, 0,
 			      mavsdk::Failure::Result::Success);
 
 
-	std::this_thread::sleep_for(std::chrono::seconds(10));
+	tester.sleep_for(std::chrono::seconds(10));
 
 	tester.check_airspeed_is_invalid(); // it's enough to check once after landing, as invalidation is permanent
 }