control/control{client,http}: don't noise dial localhost:443 in http-only tests

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>
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