add failing test for trim() after read(.., headonly=True)

There are issues with certain Trace/Stream operations after reading files with `read(.., headonly=True)`.

I know that `headonly` is somewhat special, but we should try to have it behave exactly as a commonly read file as it can be useful when inspecting waveform metadata only.

Please don't immediately merge if fixing this, I have one or two other things to investigate and might add some more failing tests.