It seems some security software or macOS itself might be MITMing TLS
(for ScreenTime?), so don't warn unless it fails x509 validation
against system roots.
Updates #3198
Change-Id: I6ea381b5bb6385b3d51da4a1468c0d803236b7bf
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>