private: Fix memory leak in tests

Don't use g_private_new(), it's deprecated, and leaks by definition.

https://bugzilla.gnome.org/show_bug.cgi?id=711751
gerror: Don't leak memory when overwrite warning

Even though we can't always make no-leak guarantees when g_warning()
in this case we're testing this behavior in tests, and it would be
good to be able to valgrind this.

https://bugzilla.gnome.org/show_bug.cgi?id=711751
unix: Fix memory leak in unix test

https://bugzilla.gnome.org/show_bug.cgi?id=711751
asyncqueue-test: Fix leaks in tests

https://bugzilla.gnome.org/show_bug.cgi?id=711751
child-test: Fix leak in test

https://bugzilla.gnome.org/show_bug.cgi?id=711751
completion-test: Fix leaks in tests

https://bugzilla.gnome.org/show_bug.cgi?id=711751
gio-test: Fix leaks in tests

https://bugzilla.gnome.org/show_bug.cgi?id=711751
mapping-test: Fix leaks in tests

https://bugzilla.gnome.org/show_bug.cgi?id=711751
datetime: Fix leak in test

https://bugzilla.gnome.org/show_bug.cgi?id=711751
mainloop-test: Fix leaks in tests

https://bugzilla.gnome.org/show_bug.cgi?id=711751
thread-test: Fix leaks in tests

This is a test of deprecated functionality and its age is
showing. Doesn't actually do what it says. But fix leaks anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=711751
unicode-encoding: Fix leaks in test

https://bugzilla.gnome.org/show_bug.cgi?id=711751
iochannel-test: Fix leaks in test

https://bugzilla.gnome.org/show_bug.cgi?id=711751