test: check resolved generated resolv.conf in networkd-test (#3628)

* test: check resolved generated resolv.conf in networkd-test

Directly verify the contents of /run/systemd/resolve/resolv.conf instead of
/etc/resolv.conf. The latter might be a plain file or a symlink to something
else (like Debian's resolvconf output), and in these cases we cannot make
strong assumptions about the contents.

Drop the "/etc/resolv.conf is a symlink" conditions and the "resolv.conf can
have at most three nameservers" alternatives, as we know that resolved always
adds all nameservers.

Explicitly start resolved at the start of a test to ensure that it is running.

* test: get along with existing system search domains in resolv.conf

The previous change has uncovered a bug in the tests: Existing search domains
can exist in resolv.conf which test_search_domains{,_too_long} didn't take into account.

As existing domains take some of the "max 6 domains" and "max 255 chars" limit,
don't expect that the last items from our test data actually appears in the
output, just the first few.

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