cmd/go/internal/load/test: parse overlay files for test functions

The existing implementation implicitly reads from the filesystem
instead of using the overlay file data (due to src == nil), so
pass in the overlaid source if we have an overlay for this file.

Fixes #44946

Change-Id: I61ce09d10c5edac1b47332583efdcd3c1241f58a
Reviewed-on: https://go-review.googlesource.com/c/go/+/305071


Reviewed-by: Jay Conrod <jayconrod@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Trust: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Go Bot <gobot@golang.org>