Fix testifylint lint errors

[jasikpark]

https://github.com/Antonboom/testifylint contains lints for validating that you avoid pitfalls of using testify.

I fixed all of the rules except for require-error which asks you to use require.Error instead of assert.Error because it calls t.FailNow() and makes it less likely that you accidentally swallow the resulting error.

Mostly it's syntactical style and better error messages, plus fixing cases where "expected" and "actual" were switched, making error messages more confusing.

There was one case where we were testing whether an error was an error, rather than whether it matched the error in question, caught by error-is-as

[jasikpark]

fw.Drop catches the fact that h1 is initialized w/o a valid vpnIp before it actually checks the firewall rule later.

Caught by this block:

nebula/firewall.go

Lines 435 to 439 in 2b427a7

	// Simple case: Certificate has one IP and no subnets
	if fp.RemoteIP != h.vpnIp {
	f.metrics(incoming).droppedRemoteIP.Inc(1)
	return ErrInvalidRemoteIP
	}