Add --enable-installed-tests configure option

See https://live.gnome.org/GnomeGoals/InstalledTests for more
information.

The tests now support being run both uninstalled and installed, so
'make check' works for those who want it.  For tests which need data
files, the way this works is they look in the compiled in value of
SRCDIR by default, and the generated tests use "env G_TEST_DATA=" to
override that.

This patch only converts glib/tests for now; if this patch looks good,
I'll do the rest of the tests.

https://bugzilla.gnome.org/show_bug.cgi?id=699079