1eaad7d3de regressed some tests in another repo that were starting up
a control server on `http://127.0.0.1:nnn`. Because there was no https
running, and because of a bug in 1eaad7d3de (which ended up checking
the recently-dialed-control check twice in a single dial call), we
ended up forcing only the use of TLS dials in a test that only had
plaintext HTTP running.
Instead, plumb down support for explicitly disabling TLS fallbacks and
use it only when running in a test and using `http` scheme control
plane URLs to 127.0.0.1 or localhost.
This fixes the tests elsewhere.
Updates #13597
Change-Id: I97212ded21daf0bd510891a278078daec3eebaa6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
a01b545441