test-functions: make sure we use the right library path for binaries without RPATH

test-functions: make sure we use the right library path for binaries without RPATH

Meson appears to set the rpath only for some binaries it builds, but not
all. (The rules are not clear to me, but that's besides the point of
this commit).

Let's make sure if our test script operates on a binary that has no
rpath set we fall back preferably to the BUILD_DIR rather than directly
to the host.

This matters if a test uses a libsystemd symbol introduced in a version
newer than the one on the host. In that case "ldd" will not work on the
test binary if rpath is not set. With this fix that behaviour is
corrected, and "ldd" works correctly even in this case.

(Or in other words: before this fix on binaries lacking rpath we'd base
dependency info on the libraries of the host, not the buidl tree, if
they exist in both.)