Send logs from zap to t.Log()

[ash2k]

Hey, guys, check this out. Quite nice to see logs only for the failing test even if tests are run concurrently. Should use this everywhere.
Example output from another project:

--- FAIL: TestCredsForUnknownRole (0.01s)
    --- FAIL: TestCredsForUnknownRole/role_added_and_removed_after_creds_fetched (0.00s)
        utils_for_tests.go:35: 2018-03-19T10:54:41.530Z debug   Starting worker
        utils_for_tests.go:35: 2018-03-19T10:54:41.530Z debug   Starting worker
        utils_for_tests.go:35: 2018-03-19T10:54:41.530Z debug   Attempting to fetch credentials for IAM role    {"role_arn": "arn:aws:iam::123456789012:role/this/is/a/path/roleName", "session_name": "default/serviceaccounts/accountname"}
        utils_for_tests.go:35: 2018-03-19T10:54:41.530Z debug   Successfully fetched credentials for IAM role   {"role_arn": "arn:aws:iam::123456789012:role/this/is/a/path/roleName", "session_name": "default/serviceaccounts/accountname"}
        assertions.go:238: 
                        Error Trace:    prefetcher_test.go:229
                        Error:          Not equal: 
                                        expected: context.deadlineExceededError(context.deadlineExceededError{})
                                        actual  : <nil>(<nil>)
                        Test:           TestCredsForUnknownRole/role_added_and_removed_after_creds_fetched
        utils_for_tests.go:35: 2018-03-19T10:54:41.531Z debug   Stopping worker
        utils_for_tests.go:35: 2018-03-19T10:54:41.531Z debug   Stopping worker
FAIL