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/README.md

@@ -16,7 +16,7 @@ SPDX-License-Identifier: curl
   - diff (when a test fails, a diff is shown)
   - stunnel (for HTTPS and FTPS tests)
   - OpenSSH or SunSSH (for SCP, SFTP and SOCKS4/5 tests)
-  - nghttpx (for HTTP/2 tests)
+  - nghttpx (for HTTP/2 and HTTP/3 tests)
   - nroff (for --manual tests)
   - An available `en_US.UTF-8` locale
 
@@ -69,6 +69,11 @@ SPDX-License-Identifier: curl
 
   The HTTP server supports listening on a Unix domain socket, the default
   location is 'http.sock'.
+  
+  For HTTP/2 and HTTP/3 testing an installed `nghttpx` is used. HTTP/3
+  tests check if nghttpx supports the protocol. To override the nghttpx
+  used, set the environment variable `NGHTTPX`. The default can also be
+  changed by specifying `--with-test-nghttpx=<path>` as argument to `configure`.
 
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