Fix broken main: acceptance source-gen harness leaks nested bin/obj into multi-project asset builds#9504

Evangelink merged 2 commits into

mainmicrosoft/testfx:mainfrom

dev/amauryleve/fix-assemblyfixtureprovider-sourcegen-bumicrosoft/testfx:dev/amauryleve/fix-assemblyfixtureprovider-sourcegen-buCopy head branch name to clipboard