Unify the way to find dbus-daemon test binary

Unify the way to find dbus-daemon test binary

There are two ways to find the dbus-daemon for testing. The first one is
defined as string at compile stage and the second one is export it from
test environment.

The first way has limitation that after defined, it's static string, so
it's impossible to run installable check. So let's unify to the second
way.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=37849
[added missing "}" -smcv]
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>