test: improve handling of ASan under clang

test: improve handling of ASan under clang

Running integration tests with ASan is somewhat tricky to begin with, as
we need to pre-load the ASan runtime DSO for certain services (like
dbus), otherwise they won't start or behave as expected. In case of gcc
this is pretty easy, as we need the runtime DSO during compilation, so
it's already present on the host system. For clang things get more
complicated, as ASan is compiled in statically by default, thus to
enable the necessary dynamic-ish behavior one needs to compile with
-shared-libasan and then correctly set LD_PRELOAD_PATH, as the runtime
libraries are not in a standard library path.