crypto/x509: ignore harmless edge case in TestSystemRoots

The no-cgo validation hack lets in certificates from the root store that
are not marked as roots themselves, but are signed by a root; the cgo
path correctly excludes them. When TestSystemRoots compares cgo and
no-cgo results it tries to ignore them by ignoring certificates which
pass validation, but expired certificates were failing validation.

Letting through expired certs is harmless anyway because we will refuse
to build chains to them.

Fixes #29497

Change-Id: I341e50c0f3426de2763468672f9ba1d13ad6cfba
Reviewed-on: https://go-review.googlesource.com/c/156330


Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>