pytester: test for _makefile joining an absolute path
pytester: test for _makefile joining an absolute path

Ref: https://github.com/pytest-dev/pytest/pull/6578#discussion_r371035867

The-Compiler/pytest,nicoddemus/pytest,markshao/pytest,pytest-dev/pytest,alfredodeza/pytest,RonnyPfannschmidt/pytest,The-Compiler/pytest,Akasurde/pytest,nicoddemus/pytest