Auto merge of #59766 - xales:revertlibtest, r=Manishearth
Revert "Auto merge of #57842 - gnzlbg:extract_libtest, r=gnzlbg"
This reverts commit 3eb4890dfe6db0279fdd3cda19f9643873ae3db9, reversing
changes made to 7a4df3b53da369110984a2b57419c05a53e33b38.
This is, as best I can tell, a clean revert of #57842. It retains some interim changes, like moving `black_box`, and otherwise brings libtest back in as it was before removal.