Remove goto usage from Poller tests

When a test fails, the line where the call to Fail occured is shown to the user.
If goto is used, the error message is misleading, because the whole test shares
the same call to Fail. It is thus not possible to say where the error occured.

This commit replaces goto usage with immediate calls to Error and Fatal.

Signed-off-by: Ondrej Kupka <ondra.cap@gmail.com>