Auto merge of #135112 - tgross35:combine-select-unpredictable-test, r=the8472

Merge the intrinsic and user tests for `select_unpredictable`

[1] mentions that having a single test with `-Zmerge-functions=disabled` is preferable to having two separate tests.  Apply that to the new `select_unpredictable` test here.

[1]: rust-lang/rust#133964 (comment)

Author: bors