tests: add HTTP/3 test case, custom location for proper nghttpx

- adding support for HTTP/3 test cases via a nghttpx server that is
  build with ngtcp2 and nghttp3.
- test2500 is the first test case, performing a simple GET.
- nghttpx is checked for support and the 'feature' nghttpx-h3
  is set accordingly. test2500 will only run, when supported.
- a specific nghttpx location can be given in the environment
  variable NGHTTPX or via the configure option
    --with-test-nghttpx=<path>

Extend NGHTTPX config to H2 tests as well

* use $ENV{NGHTTPX} and the configured default also in http2 server starts
* always provide the empty test/nghttpx.conf to nghttpx. as it defaults to
  reading /etc/nghttpx/nghttpx.conf otherwise.

Added nghttpx to CI ngtcp2 jobs to run h3 tests.

Closes #9031

tests/data/Makefile.inc

@@ -243,6 +243,8 @@ test2200 test2201 test2202 test2203 test2204 test2205 \
 \
 test2300 test2301 test2302 test2303 \
 \
+test2500 \
+\
 test3000 test3001 test3002 test3003 test3004 test3005 test3006 test3007 \
 test3008 test3009 test3010 test3011 test3012 test3013 test3014 test3015 \
 test3016 test3017 test3018 test3019 test3020 test3021 test3022 test3023 \

tests/data/test2500

@@ -0,0 +1,78 @@
+<testcase http-crlf="yes">
+<info>
+<keywords>
+HTTP
+HTTP GET
+HTTP/3
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<data nocheck="yes">
+HTTP/1.1 200 OK
+Date: Tue, 09 Nov 2010 14:49:00 GMT
+Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
+ETag: "21025-dc7-39462498"
+Accept-Ranges: bytes
+Content-Length: 6
+Connection: close
+Content-Type: text/html
+Funny-head: yesyes
+
+-foo-
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<features>
+http/3
+nghttpx-h3
+</features>
+<server>
+http
+http/3
+</server>
+ <name>
+HTTP/3 GET:
+ </name>
+ <command>
+--cacert %SRCDIR/certs/EdelCurlRoot-ca.crt --http3 --resolve localhost:%HTTP3PORT:%HOSTIP https://localhost:%HTTP3PORT/%TESTNUMBER
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<strip>
+^X-Forwarded-Proto:.*
+^Via:.*
+</strip>
+<protocol>
+GET https://localhost:%HTTP3PORT/%TESTNUMBER HTTP/1.1
+Host: localhost:%HTTP3PORT
+User-Agent: curl/%VERSION
+Accept: */*
+
+</protocol>
+<stdout crlf="yes">
+HTTP/3 200 
+date: Tue, 09 Nov 2010 14:49:00 GMT
+last-modified: Tue, 13 Jun 2000 12:10:00 GMT
+etag: "21025-dc7-39462498"
+accept-ranges: bytes
+content-length: 6
+content-type: text/html
+funny-head: yesyes
+via: 1.1 nghttpx
+
+-foo-
+</stdout>
+<stripfile>
+s/^server: nghttpx.*\r?\n//
+</stripfile>
+</verify>
+</testcase>