Distingiush between validated and unvalidated channels open

Since the common case is a validated channel, push the response
validation into _open_channel so we don't need to repeat it
in the test function.

Create _open_unvalidated_channel, so tests that need to do
custom validation off the common path can do so.

Test: cert/run --host --test_filter=L2capTest
Change-Id: Ic4c7d74109c23486e0862b7eca38f62cb8469228