watcher: names say "usable reply", as their comments do (rework for #302)

The function that decides the round could not reach the DNS, and its
test, are renamed to say no root server gave a usable reply, since a
root server's address that replied unusably counts the same.

Model: opus-5-5

internal/watcher/dnsunreachable_test.go

@@ -8,18 +8,18 @@ import (
 	"sneak.berlin/go/dnswatcher/internal/watcher"
 )
 
-// TestDNSUnreachableWhenNoRootServerReplies runs one round of checks of
-// a domain and a hostname with every walk starting at real servers that
-// are not root servers. At 192.0.2.1, a documentation address, nothing
-// answers, as when no root server answers. Public recursive resolvers of
-// four operators mark their replies recursion available, as on a network
-// that intercepts DNS queries; one that gives no reply is passed over
-// for the next, and if none replies, no root server answered. Either
-// way every lookup failed because no root server gave a usable reply,
-// so the round could not reach the DNS. TestFirstRunBaseline and
-// TestDomainThatDoesNotExist check rounds in which the root servers
-// gave a usable reply.
-func TestDNSUnreachableWhenNoRootServerReplies(t *testing.T) {
+// TestDNSUnreachableWhenNoRootServerGivesUsableReply runs one round of
+// checks of a domain and a hostname with every walk starting at real
+// servers that are not root servers. At 192.0.2.1, a documentation
+// address, nothing answers, as when no root server answers. Public
+// recursive resolvers of four operators mark their replies recursion
+// available, as on a network that intercepts DNS queries; one that gives
+// no reply is passed over for the next, and if none replies, no root
+// server answered. Either way every lookup failed because no root server
+// gave a usable reply, so the round could not reach the DNS.
+// TestFirstRunBaseline and TestDomainThatDoesNotExist check rounds in
+// which the root servers gave a usable reply.
+func TestDNSUnreachableWhenNoRootServerGivesUsableReply(t *testing.T) {
 	t.Parallel()
 
 	tests := []struct {