[Libguestfs] [PATCH nbdkit] tests: Remove QEMU_IO / HAVE_QEMU_IO.

Currently we test for qemu-img, socat, ss, certtool, etc at run time, but we test for qemu-io at compile time (in ./configure). This commit removes this inconsistency. I would consider the opposite patch (which makes qemu-img etc tested at configure time). The main advantage of testing for these binaries at run time is that tests are not "silently" omitted. Instead tests with insufficient dependencies are now reported as ‘SKIP’, which I think gives packagers a better overview of their coverage of the test suite. I recently changed the Fedora build so we run the test suite on every architecture, and it's interesting that x86_64 is now running and passing 40 tests, but some architectures (s390x for instance) only manage 17 passes with the rest being skipped for multiple reasons. Disadvantages: - Can't override the location of these binaries eg by setting QEMU_IO=/opt/qemu/bin/qemu-io ./configure - ./configure doesn't report missing soft dependencies. There are also other test dependencies that we test at compile time but we cannot change that (guestfish, libguestfs). Rich.