Don't override `testthat::expect_equal()` in helper

[m-muecke]

Since basically all the mlr3 repos are using testthat version 3 it would be easier for using the helper test functions if the testthat::expect_equal() is not overriden to its v2 behaviour in the following file: https://github.com/mlr-org/mlr3pipelines/blob/master/inst/testthat/helper_compat.R