It's only used in test code. We have to put it in its own translation
unit with no non-libc dependencies so that we can compile a copy of it
without AddressSanitizer support, because in a subsequent commit we will
special-case test-segfault to be compiled without using AddressSanitizer,
which would make linking to an AddressSanitizer-instrumented libdbus fail.

Signed-off-by: Simon McVittie <smcv@collabora.com>