Modify gio/tests/gsettings.c test to create files in a temporary directory

Currently, gio/tests/gsettings.c creates various files it needs for the tests in the current directory, which is not a very tidy way of doing things. Specifically, it means that Makefile.am creates a CLEANFILES list to clean those files up again before distcheck will pass.

Modify the test to create a temporary directory, create and use all its files in there, and then delete the directory before successfully exiting.