Rollup merge of #144909 - GuillaumeGomez:extend-libtest-for-merged-doctests, r=Amanieu

Add new `test::print_merged_doctests_times` used by rustdoc to display more detailed time information

This PR is the first commit of rust-lang/rust#144908. Sadly, it seems like we can't do all of it at once...

r? ``@Amanieu``

Author: Zalathar