test1658: add unit test for the HTTPS RR decoder

Made the HTTPS-RR parser a little stricter while at it.

Drop the ALPN escape handling, that was not needed.

Make the hode handle (and ignore) duplicate ALPN entries.

Closes #16972

tests/data/Makefile.am

@@ -220,6 +220,7 @@ test1620 test1621 \
 test1630 test1631 test1632 test1633 test1634 test1635 \
 \
 test1650 test1651 test1652 test1653 test1654 test1655 test1656 test1657 \
+test1658 \
 test1660 test1661 test1662 test1663 test1664 \
 \
 test1670 test1671 \

tests/data/test1658

@@ -0,0 +1,30 @@
+<testcase>
+<info>
+<keywords>
+unittest
+doh
+httpsrr
+</keywords>
+</info>
+
+#
+# Client-side
+<client>
+<server>
+none
+</server>
+<features>
+unittest
+DoH
+</features>
+<name>
+unit test for doh_resp_decode_httpsrr
+</name>
+</client>
+<verify>
+<stderr mode="text">
+URL: -
+Test ended with result 0
+</stderr>
+</verify>
+</testcase>