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>
diff --git a/meson.build b/meson.build
index f3f7b9c..f025623 100644
--- a/meson.build
+++ b/meson.build
@@ -3,9 +3,9 @@
                           'b_staticpic=false', 'stdsplit=false'],
         version: files('VERSION'))
 
-add_test_setup('quick', exclude_suites: ['slow', 'thorough'], is_default: true)
-add_test_setup('slow', exclude_suites: ['thorough'], env: ['G_TEST_SLOW=1', 'SPEED=slow'])
-add_test_setup('thorough', env: ['G_TEST_SLOW=1', 'SPEED=thorough'])
+add_test_setup('quick', exclude_suites: ['block', 'slow', 'thorough'], is_default: true)
+add_test_setup('slow', exclude_suites: ['block', 'thorough'], env: ['G_TEST_SLOW=1', 'SPEED=slow'])
+add_test_setup('thorough', exclude_suites: ['block'], env: ['G_TEST_SLOW=1', 'SPEED=thorough'])
 
 not_found = dependency('', required: false)
 keyval = import('keyval')