gnu: gnucash: Disable the stress-options-test using a phase.

The qof test is also reinstated; it was failing because the needed locales
were not installed.

* gnu/packages/patches/gnucash-disable-failing-tests.patch: Delete file.
* gnu/local.mk (dist_patch_DATA): Unregister it.
* gnu/packages/gnucash.scm (gnucash)[patches]: Remove it.
[phases]{disable-stress-options-test}: Implement it using SUBSTITUTES*.
{install-locales}: New phase.