Translate Python-based tests to C

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 OS services like it can on most Linux systems).

This branch also changes test-service to write diagnostics to stderr. When run by a TAP test, stdout is reserved for machine-readable TAP output.