Mark curl tests using http2.golang.org as XFAIL

http2.golang.org/serverpush has been retired[1], so we need to come up
with an alternative.  Until then, we mark the relevant tests as XFAIL
(although bug77535.phpt passes, what might be an indication that the
test needs further revision).  To avoid waiting for the timeout, we
also unconditionally skip these tests for now.

[1] <golang/go#49301>

Closes GH-7829.

ext/curl/tests/bug76675.phpt

@@ -1,5 +1,7 @@
 --TEST--
 Bug #76675 (Segfault with H2 server push write/writeheader handlers)
+--XFAIL--
+http2.golang.org/serverpush is gone
 --SKIPIF--
 <?php
 include 'skipif.inc';
@@ -10,6 +12,7 @@ $curl_version = curl_version();
 if ($curl_version['version_number'] < 0x073d00) {
     exit("skip: test may crash with curl < 7.61.0");
 }
+die("skip test is slow due to timeout, and XFAILs anyway");
 ?>
 --FILE--
 <?php

ext/curl/tests/bug77535.phpt

@@ -1,5 +1,7 @@
 --TEST--
 Bug #77535 (Invalid callback, h2 server push)
+--XFAIL--
+http2.golang.org/serverpush is gone
 --SKIPIF--
 <?php
 include 'skipif.inc';
@@ -10,6 +12,7 @@ $curl_version = curl_version();
 if ($curl_version['version_number'] < 0x073d00) {
     exit("skip: test may crash with curl < 7.61.0");
 }
+die("skip test is slow due to timeout, and XFAILs anyway");
 ?>
 --FILE--
 <?php