tests: add expect_protocol_error function

tests: add expect_protocol_error function

This function checks if a particular protocol error came in wire.
It's usefull in the cases where we hitherto used FAIL_TEST.
The problem with FAIL_TEST is that *any* assert will pass the test,
but we want only some asserts to pass the test (i. e. we don't
want the test to pass when it, for example, can't connect to display).
FAIL_TESTs are good only for sanity testing.

The expect_protocol_error allows us to turn all FAIL_TESTs to TESTs
as will be introduced in following patches.

v2: fixed white-space error and a mistake in comment

Reviewed-by: Bryce Harrington <b.harrington@samsung.com>
Reviewed-by: Pekka Paalanen <ppaalanen@gmail.com>