This fixes the implementation and test from #15208 which apparently
never worked.
Ignore the metacert when counting the number of expected certs
presented.
And fix the test, pulling out the TLSConfig setup code into something
shared between the real cmd/derper and the test.
Fixes#15579
Change-Id: I90526e38e59f89b480629b415f00587b107de10a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Brad Fitzpatrick