build-sys: Do not build tests that depend on resolved when it has been disabled

build-sys: Do not build tests that depend on resolved when it has been disabled

If ./configure --disable-resolved has been used, do not try to build
test-dns-packet and test-resolve-tables which depend on it.

Previously, the SOURCES, LIBS and LDADDs for these tests were made conditional
while the main rules for them weren't, causing build failures trying to build a
binary with no sources.

This was uncovered when trying to build udeb for systemd in CI, which uses
--disable-resolved for a minimal build, which uncovered the issue.

Fixes #2651.