This simplifies bootstrapping: now you don't have to build dbus,
build dbus-python (with GLib), and use dbus-python to test dbus.

It also avoids test failures when using facilities like
AddressSanitizer. When libdbus is built with AddressSanitizer, but the
system copies of Python and dbus-python were not, dbus-python will exit
the Python interpreter on load, because libasan wasn't already
initialized. The simplest way to avoid this is to not use Python:
the scripts are not *that* hard to translate into C.

Both of these tests happen to be conditionally compiled for Unix only.
test_activation_forking() relies on code in TestSuiteForkingEchoService
that calls fork(), which can only work on Unix; meanwhile,
test_system_signals() tests the system bus configuration, which is
only relevant to Unix because we don't support using dbus-daemon as
a privilege boundary on Windows (and in any case D-Bus is not a Windows
OS feature, so the system bus cannot be used to communicate with...