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run connection gating tests on both TCP and QUIC

Also disables one test that doesn't work with QUIC, as we always
complete the handshake before gating there.

fix: avoid returning typed nils

run go mod tidy

fix staticcheck

fix go vet

New Dialer

Fix simultaneous dials

they work with private data types, so there is no point in having them public

so that we exercise the dialWorker dial to self error path

we might get more connections because simultaneous dials can succeed
and we have both TCP and QUIC addrs by default

for consistency with the old dialer behaviour.

i want to use binary literals; technically only requires 1.13, but let's not be ancient

makes TestDialBackoff happy