ekf_unit: reduce init period

Fake fusion is when at rest is quite strong and makes the variance
reduce rapidly. Reduce the amount of time we wait before checking if the
varicances are still large enough.

src/modules/ekf2/test/test_EKF_initialization.cpp

@@ -50,7 +50,7 @@ public:
 	SensorSimulator _sensor_simulator;
 	EkfWrapper _ekf_wrapper;
 
-	const float _init_tilt_period = 1.0; // seconds
+	const float _init_tilt_period = 0.3f; // seconds
 
 	// GTests is calling this
 	void SetUp() override