Fix running tests with jpeg, tiff and zlib support being unavailable

Some of the current tests are skipped properly when relevant features are not available. This patch adds similar measures to all the remaining tests, so that the test suite succeeds even with all features being disabled.

In most cases, this was done through conditional test skipping. `test_imagefile` is done only on the formats that are enabled. `test_image_draft` gets a fallback to a PCX file (and no RGBA) if PNG is not available.