Demonstrate that beatgrid implementation is broken

[Holzhaus]

Documentation from the Beats class header file:

    // Starting from sample dSamples, return the sample of the next beat in the
    // track, or -1 if none exists. If dSamples refers to the location of a
    // beat, dSamples is returned.
    virtual double findNextBeat(double dSamples) const = 0;

    // Starting from sample dSamples, return the sample of the previous beat in
    // the track, or -1 if none exists. If dSamples refers to the location of
    // beat, dSamples is returned.
    virtual double findPrevBeat(double dSamples) const = 0;

If the code actually behaves as described here, these DEBUG_ASSERT statements should not be a problem. But they make tests fail on my machine.

[github-actions]

This PR is marked as stale because it has been open 90 days with no activity.

[Holzhaus]

Fixed in #4099.