build: make check-block a meson test

build: make check-block a meson test

"meson test" can be asked to run tests verbosely; this makes it usable
also for qemu-iotests's own harness, and it lets "make check-block"
reuse mtest2make.py's infrastructure to find and build test dependencies.

Adjust check-block.sh to use the standard exit code that reports a test
as skipped.  Alternatively, in the future we could make it produce TAP
output, which is consistent with all other "make check" tests.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>