build: Add --disable-modular-tests build option

This patch solves two problems:

First, it allows builders to optionally cut the circular dependency
between dbus and glib by disabling the modular tests (just like how
the tests can be disabled in dbus).

Second, the tests are entirely pointless to build if cross-compiling.

It also moves us slightly closer to the long term future we want where
the tests are a separate ./configure invocation and run against the
INSTALLED glib, not the one in the source tree. This would allow us to
run the tests constantly, not just when glib is built.

https://bugzilla.gnome.org/show_bug.cgi?id=667806