[MRG] Make random_state test for IterativeImputer behave as expected

[leo-mazz]

Reference Issues/PRs

Fixes a test that was not implemented properly in #15636 (see also #15611).

What does this implement/fix? Explain your changes.

• ENH Do not pass random_state to estimator in IterativeImputer #15636 added a test ensure IterativeImputer did not overwrite the random state of its estimator, but the test was not implemented correctly, and would trivially pass. This PR fixes that
• Updated note in the user guide on experimental status of IterativeImputer, to remove reference to an issue that's already closed (Consider using integer random_state in IterativeImputer #15611)

Other comments

This is my first PR to an open source project, I apologise if I haven't done this properly.

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