[Transform] reenable warning checks in pivot tests

[hendrikmuhs]

reenable warning checks(deprecation warnings) in pivot tests

Note:

• this is a leftover when the old interval was deprecated and split into fixed and calendar in Force selection of calendar or fixed intervals in date histo agg #33727, transforms adjusted in [ML-DataFrame] use calendar|fixed_interval in favor of interval #42297, but the to be removed loc stayed
• [ML-DataFrame] use calendar|fixed_interval in favor of interval #42297 removed the support of the old interval, therefore the implementation can't run into deprecation warnings for the old interval parameter

[elasticmachine]

Pinging @elastic/ml-core (:ml/Transform)

[przemekwitek]

LGTM