hamamatsu: Check for empty MapFile key

Mathieu Malaterre <mathieu.malaterre@gmail.com> reported a VMU slide
with an empty MapFile key:

    http://lists.andrew.cmu.edu/pipermail/openslide-users/2012-December/000540.html

Explicitly test for this case and produce a sane error message.
hamamatsu: Support empty MacroImage value

Mathieu Malaterre reported a VMU slide with an empty MacroImage value,
which fails to open with:

    Could not read macro image: Empty input file

Don't choke on this case.
hamamatsu: Fix use-after-free in VMU error path
test/try_open: Add some helper functions
test: Allow test cases to read regions and check properties
test: Check quickhash-1 for each slide

to ensure the hashes are not inadvertently changed.