samples: sockets: Fix syntax of combined "filter" clauses in sample.yaml

Now that we have support for properly combine conditions from multiple
"filter" clauses in one sample.yaml file (e.g. from "common" and
test-specific sections), use it instead of previous adhoc syntax
relying on string concatenation.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>