net/tlsdial: fix TLS cert validation of HTTPS proxies

If you had HTTPS_PROXY=https://some-valid-cert.example.com running a
CONNECT proxy, we should've been able to do a TLS CONNECT request to
e.g. controlplane.tailscale.com:443 through that, and I'm pretty sure
it used to work, but refactorings and lack of integration tests made
it regress.

It probably regressed when we added the baked-in LetsEncrypt root cert
validation fallback code, which was testing against the wrong hostname
(the ultimate one, not the one which we were being asked to validate)

Fixes #16222

Change-Id: If014e395f830e2f87f056f588edacad5c15e91bc
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/tlstest/testdata/root-ca.key

@@ -0,0 +1,5 @@
+-----BEGIN EC PRIVATE KEY-----
+MHcCAQEEIMl3xjqt1dnXBpYJSEqevirAcnSJ79I2tucdRazlrDG9oAoGCCqGSM49
+AwEHoUQDQgAEQ/+Jme+16hgO7TtPSIFHVV0Yt969ltVlARVcNUZmWc0upQaq7uiJ
+Aur5KtzwxU3YI4bhNK0593OK2TLvEEWIdw==
+-----END EC PRIVATE KEY-----