sanitycheck: Merge common vs per-test "filter" field values semantically

Currently, string values comming from "common" test definition section
and from test-specific section are just concatenated. Suppose, we want
to define some common filter condition, and also per-test additional
criteria. Currently, that leads to following syntax:

common:
  filter: TOOLCHAIN_FOO == 1
tests:
  sample.net.sockets.http_get.posix:
    filter: and not CONFIG_BAR

That's arguable quite adhoc, and the only way to figure it out for
most people will be to add debug logging.

This patch proposes to use the expected syntax (i.e.
"filter: not CONFIG_BAR"), and combine conditions properly based on
their semantic meaning (which also includes parans for proper
evaluation order).

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