beets-alternatives: Fix tests against beets 1.4.6

Since the bump of beets to version 1.4.6 in e5fab33e


the tests no longer run successfully because beets 1.4.6 introduces a
breaking API change for the Item.move() method which now instead of just
passing copy=True the operation is now passed using a different
"operation" keyword argument.

Unfortunately the original repository of beets-alternatives is
unmaintained since 3 years and thus there is no upstream fix available
at the moment.

However, there is a fork maintained by @wisp3rwind, which addresses this
problem (wisp3rwind/beets-alternatives@33c6525ed4f799a2a4991c9fc9d55524)
and a bunch of other fixes.

The reason why I'm not using the patch from @wisp3rwind is that it
simply doesn't apply against beets-alternatives 0.8.2, but my patch here
essentially does the same.

Signed-off-by: aszlig <aszlig@nix.build>
Upstream issue: geigerzaehler/beets-alternatives#13
Cc: @Profpatsch