tests/domaincaps: Use testQemuCapsIterate()

Now that the only data we need for fully testing a QEMU binary is
the (version, arch) combo, we can stop providing that information
ourselves and instead rely on testQemuCapsIterate() automatically
picking up new input files as they are added to the repository,
the same way the qemucapabilities and qemucaps2xml tests already
behave.

Unsurprisingly, this change results in a bunch of extra output
files being created, significantly expanding our test coverage.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Jiri Denemark <jdenemar@redhat.com>