Fix bug with incorrect thrust direction and update test for vel and x_cg

src/forces.zig

@@ -11,7 +11,7 @@ pub fn propThrust(attitude: vec3.QuatF64, p: drone.Propeller) struct { vec3.Vec3
     const body_thrust = vec3.Vec3F64.init(
         0,
         0,
-        p.thrust_coefficient * p.angular_velocity * p.angular_velocity,
+        -p.thrust_coefficient * p.angular_velocity * p.angular_velocity,
     );
 
     const force = vec3.quatApply(attitude, body_thrust);
@@ -57,7 +57,7 @@ test "thrust is correct" {
         .moment_coefficient = 2,
     };
 
-    const expected_thrust_mag = 100 * 100;
+    const expected_thrust_mag = -100 * 100;
     const thrust, const momentum = propThrust(attitude, prop);
 
     try std.testing.expectApproxEqAbs(0, thrust.x(), 1e-11);