During build-time testing, we can use `$<TARGET_FILE:childprocess>` to
pass the just-built childprocess executable to it as an argument, but
during "as-installed" testing we need to use the installed path of
the childprocess executable instead.
Signed-off-by: Simon McVittie <smcv@debian.org>
Otherwise they'll try to run with no arguments, and fail, when run by
ginsttest-runner.
Fixes: 81ca9d61 "cmake+test: add more automated tests + use properties"
Signed-off-by: Simon McVittie <smcv@collabora.com>