os/exec: don't run TestExtraFiles if extra files were open for the test

os/exec: don't run TestExtraFiles if extra files were open for the test

Our attempts to close existing open files are flaky. They will fail if,
for example, file descriptor 3 is open when the test binary starts.
Instead, report any such cases, and skip TestExtraFiles.

Updates #35469

Change-Id: I7caec083f3f4a31579bf28fc9c82ae89b1bde49a
Reviewed-on: https://go-review.googlesource.com/c/go/+/206939
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>