tests/functional: Add a decorator for skipping long running tests

tests/functional: Add a decorator for skipping long running tests

Some tests have a very long runtime and might run into timeout issues
e.g. when QEMU has been compiled with --enable-debug. Add a decorator
for marking them more easily. Rename the corresponding environment
variable to be more in sync with the other QEMU_TEST_ALLOW_* switches
that we already have, and add a paragraph about it in the documentation.

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-ID: <20250128152839.184599-2-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>