os/exec: re-enable TestExtraFiles checks skipped on various OSes

os/exec: re-enable TestExtraFiles checks skipped on various OSes

The issues associated with these skipped checks are closed.
If they are working around unfixed bugs, the issues should remain open.
If they are working around unfixable properties of the system, the skips
should refer to those properties rather than closed issues.

Updates #2603
Updates #3955
Updates #25628

Change-Id: I3491c69b2ef5bad0fb12001fe8f7e06b424883ca
Reviewed-on: https://go-review.googlesource.com/c/go/+/201718
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>