tests/qtest/migration: Move a couple of slow tests under g_test_slow

The xbzrel and vcpu_dirty_limit are the two slowest tests from
migration-test. Move them under g_test_slow() to save about 40s per
run.

Signed-off-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240911145204.17692-1-farosas@suse.de
Signed-off-by: Peter Xu <peterx@redhat.com>

tests/qtest/migration-test.c

@@ -3803,8 +3803,10 @@ int main(int argc, char **argv)
 
     migration_test_add("/migration/precopy/unix/plain",
                        test_precopy_unix_plain);
-    migration_test_add("/migration/precopy/unix/xbzrle",
-                       test_precopy_unix_xbzrle);
+    if (g_test_slow()) {
+        migration_test_add("/migration/precopy/unix/xbzrle",
+                           test_precopy_unix_xbzrle);
+    }
     migration_test_add("/migration/precopy/file",
                        test_precopy_file);
     migration_test_add("/migration/precopy/file/offset",
@@ -3979,7 +3981,7 @@ int main(int argc, char **argv)
     if (g_str_equal(arch, "x86_64") && has_kvm && kvm_dirty_ring_supported()) {
         migration_test_add("/migration/dirty_ring",
                            test_precopy_unix_dirty_ring);
-        if (qtest_has_machine("pc")) {
+        if (qtest_has_machine("pc") && g_test_slow()) {
             migration_test_add("/migration/vcpu_dirty_limit",
                                test_vcpu_dirty_limit);
         }