promis/plugin/check_free_disk_space: test: Fix unclosed file ResourceWarning
folder_disk_test.sql was opened to be read but was not closed:
slapos.toolbox$ python -m unittest discover -v -s slapos/test/promise/plugin/ -t . -k test_display_prediction
test_display_prediction (slapos.test.promise.plugin.test_check_free_disk_space.testcheckfreediskspace.test_display_prediction) ...
/usr/lib/python3.11/unittest/case.py:576: resourcewarning: unclosed file <_io.textiowrapper name='/home/kirr/src/wendelin/slapos/slapos.toolbox/slapos/test/promise/data/disktest.sql' mode='r' encoding='utf-8'>
self.setup()