As a work-around, I can include <Servo.h> in the .ino file before including the .h file.
This doesn't explain the behaviour, but it does allow me to continue my work in the meantime.
It acts as if the include path for the .h file is different than the .ino file.
#include <Servo.h> // <-------
#include "test.h"
A a;
void setup() {}
void loop() {}