crypto/x509: fix broken tests

crypto/x509: fix broken tests

Convert TestUnknownAuthorityError to use subtests, avoiding continuing
the test after an unrecoverable failure.

Skip TestIssue51759 on pre-macOS 11 builders, which don't enforce the
behavior we were testing for.

Updates #58791
Fixes #58812

Change-Id: I4e3e5bc371aa139d38052184c8232f8cb564138f
Reviewed-on: https://go-review.googlesource.com/c/go/+/472496
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Roland Shoemaker <roland@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>