Ramsey and echo tests contain values that are illegal in Qblox backend

Example:

    def setup_class(cls):
        set_datadir(tmp_dir.name)
        cls.sched_kwargs = {
            "times": np.linspace(0, 80e-6, 21),
            "qubit": "q0",
        }

        cls.sched = ts.echo_sched(**cls.sched_kwargs)
        cls.sched = qcompile(cls.sched, DEVICE_CFG)

This causes illegal wait times to occur in the backend, because the wait times get set to values that are too small for the first few points (due to the np.linspace(0, 80e-6, 21)).

This can be solved by replacing the 0 by a larger value. In the old pulsar backend this silently fails, but in the new backend an exception is raised instead (so this is blocking !81 (merged)).