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.

Makefile.am

@@ -1476,8 +1476,6 @@ tests += \
 	test-af-list \
 	test-arphrd-list \
 	test-dns-domain \
-	test-dns-packet \
-	test-resolve-tables \
 	test-install-root \
 	test-rlimit-util \
 	test-signal-util
@@ -5208,6 +5206,8 @@ bin_PROGRAMS += \
 	systemd-resolve
 
 tests += \
+	test-dns-packet \
+	test-resolve-tables \
 	test-dns-domain \
 	test-dnssec