patch 9.1.1393: missing test for switching buffers and reusing curbuf

patch 9.1.1393: missing test for switching buffers and reusing curbuf

Problem:  The check in buf_freeall that restores curwin subtly prevents
          leaving an unloaded buffer in a window when reusing curbuf, if
          autocommands switch to a different buffer.
Solution: Add a test case that covers this. Also ensure splitting isn't
          possible, as that could do the same (Sean Dewar)

closes: #17325

Signed-off-by: Sean Dewar <6256228+seandewar@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>