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/getpart.pm

@@ -41,6 +41,21 @@ sub decode_hex {
     return $s;
 }
 
+sub testcaseattr {
+    my %hash;
+    for(@xml) {
+        if(($_ =~ /^ *\<testcase ([^>]*)/)) {
+            my $attr=$1;
+            while($attr =~ s/ *([^=]*)= *(\"([^\"]*)\"|([^\> ]*))//) {
+                my ($var, $cont)=($1, $2);
+                $cont =~ s/^\"(.*)\"$/$1/;
+                $hash{$var}=$cont;
+            }
+        }
+    }
+    return %hash;
+}
+
 sub getpartattr {
     # if $part is undefined (ie only one argument) then
     # return the attributes of the section