tests: drivers: counter: basic_api: Tweak test timing

For slow processors it is possible that setting 1 ms alarm and
then canceling it could lead to alarm expiring before it is canceled
and that was not expected in the test. Increasing timeout to 1.5ms.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>