tstest/tlstest: simplify, don't even bake in any keys

I earlier thought this saved a second of CPU even on a fast machine,
but I think when I was previously measuring, I still had a 4096 bit
RSA key being generated in the code I was measuring.

Measuring again for this, it's plenty fast.

Prep for using this package more, for derp, etc.

Updates #16315

Change-Id: I4c9008efa9aa88a3d65409d6ffd7b3807f4d75e9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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