crypto/x509: cleanup name constraint tests

crypto/x509: cleanup name constraint tests

Make TestConstraintCases a bit clearer by adding actual subtest names,
mostly taken from the old comments. Also add a handful of extra test
cases.

Change-Id: Ie759d1ea85a353aeacab267bb6e175a90f20702c
Reviewed-on: https://go-review.googlesource.com/c/go/+/722481
Reviewed-by: Daniel McCarney <daniel@binaryparadox.net>
Auto-Submit: Roland Shoemaker <roland@golang.org>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>