[pickers] Fix date and time merging to retain milliseconds

[LukasTy]

Fixes tests for some reason only failing with mui@material v6 seen in #14142.

• The util had missing milliseconds retaining.
• A test was incorrect and not testing anything. 🤷 🙈

[mui-bot]

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[flaviendelangle]

One test seems to be complaining
But other than that, the fix and the test changes make a lot of sense!

[LukasTy]

One test seems to be complaining
But other than that, the fix and the test changes make a lot of sense!

Yeah, it seems that the tests was relying on a "buggy" previous behavior, where milliseconds were ignored.
I've aligned the initial and expected values to avoid the error. 👍

[flaviendelangle]

Thanks for taking care of it!