tests: ztest: ztest_mock to support multiple calls to same mock

Updated ztest_mock.c to support multiple calls to same mock function
within a single function under test. This allows sequencing mock
return values for improved decision coverage in a test, or simply
when a given function under test calls the same function more than
once with different parameters, or different return values.

Signed-off-by: Morten Priess <mtpr@oticon.com>