dovecot: Add tests for more error cases

This patch adds more tests for the dovecot library, in particular:
 - Protocol errors (invalid versions, etc.).
 - Invalid command (cli-specific test).
 - Connection breakups.

internal/dovecot/dovecot_test.go

@@ -117,3 +117,9 @@ func mustListen(t *testing.T, path string) *net.UnixListener {
 
 	return l
 }
+
+func TestNotASocket(t *testing.T) {
+	if isUnixSocket("/doesnotexist") {
+		t.Errorf("isUnixSocket(/doesnotexist) returned true")
+	}
+}