test: make runindir tests pass regardless of whether module mode is in use

The "runindir" tests used "go run", but relied on relative imports
(which are not supported by "go run" in module mode). Instead, such
tests must use fully-qualified imports, which require either a go.mod
file (in module mode) or that the package be in an appropriate
subdirectory of GOPATH/src (in GOPATH mode).

To set up such a directory, we use yet another copy of the same
overlayDir function currently found in the misc subdirectory of this
repository.

Fixes #33912
Updates #30228

Change-Id: If3d7ea2f7942ba496d98aaaf24a90bcdcf4df9f7
Reviewed-on: https://go-review.googlesource.com/c/go/+/225205


Reviewed-by: Ian Lance Taylor <iant@golang.org>