cputest: Skip tests requiring JSON_MODELS if QEMU is disabled
    
    Some tests require JSON_MODELS to be parsed into qemuCaps and applied
    when computing CPU models and such test cannot succeed if QEMU driver is
    disabled. Let's mark the tests with JSON_MODELS_REQUIRED and skip the
    appropriate parts if building without QEMU.
    
    On the other hand, CPU tests with JSON_MODELS should succeed even if
    model definitions from QEMU are not parsed and applied. Let's explicitly
    test this by repeating the tests without JSON_MODELS set.
    
    This fixes the build with QEMU driver disabled, e.g., on some
    architectures on RHEL/CentOS.
    
    Signed-off-by: Jiri Denemark <jdenemar@redhat.com>
    Reviewed-by: Pavel Hrdina <phrdina@redhat.com>