test/try_open: Rename variable
test/try_open: Defer variable declarations until initialization
Document that openslide_open() can be expensive

Closes #117.
Add openslide_detect_vendor() API function

Pursuant to #126.  The documented semantics of openslide_can_open()
prevent making it much faster than openslide_open(), so we need a
lighter-weight alternative.  As a bonus, have it return the vendor name,
so that VIPS doesn't have to call openslide_open() to detect generic-tiff
slides it doesn't want to open with OpenSlide in the first place.
test: Add expected vendor string to test cases
test: Check vendor string when evaluating test cases
test/driver: Automatically set vendor when creating config.yaml
Deprecate openslide_can_open()

The documentation for openslide_can_open() says:

    Returns: If openslide_open() will succeed.

It therefore must do all of the work that openslide_open() does, except
possibly for things that cannot fail.  Thus it can't be made much faster
than openslide_open(), and also provides no more information.  We now
have a lightweight alternative in openslide_detect_vendor(), so deprecate
openslide_can_open().

Fixes #126.
test/try_open: Drop openslide_can_open() test

openslide_can_open() now uses essentially the same code paths as
openslide_open(), so it's not as important that we test it exhaustively.
It's deprecated, so we want to silence the compiler warning.  And
removing it will speed up testsuite runs.