gnucash-2.4.15: disable test suite

The test suite was broken by the GLib 2.46.0 update. If anyone finds a
patch that remedies these issues for 2.4.x, please check it in!
Otherwise, we'll eventually update our default version to 2.6.x, I
suppose.

Closes https://github.com/NixOS/nixpkgs/issues/11084.