fix: work around change in upstream libxml2 recovery (#3013)

**What problem is this PR intended to solve?**

Upstream libxml2 slightly changed encoding in recovery mode. Let's
update the one failing test, which is intended to test general recovery
mode error handling (and not encoding specifically) to be less specific
and forwards-compatible.

See:

- https://gitlab.gnome.org/GNOME/libxml2/-/issues/598
-
https://gitlab.gnome.org/GNOME/libxml2/-/commit/c082ef4644ce1f0af2a7c01e27c4a75f4d7d4feb

Author: flavorjones

test/xml/sax/test_push_parser.rb

@@ -182,12 +182,18 @@ def error(msg)
 
   it :test_broken_encoding do
     skip_unless_libxml2("ultra hard to fix for pure Java version")
+
     parser.options |= Nokogiri::XML::ParseOptions::RECOVER
     # This is ISO_8859-1:
     parser << "<?xml version='1.0' encoding='UTF-8'?><r>Gau\337</r>"
     parser.finish
+
     assert_operator(parser.document.errors.size, :>=, 1)
-    assert_equal "Gau\337", parser.document.data.join
+
+    # the interpretation of the byte may vary by libxml2 version in recovery mode
+    # see for example https://gitlab.gnome.org/GNOME/libxml2/-/issues/598
+    assert(parser.document.data.join.start_with?("Gau"))
+
     assert_equal [["r"]], parser.document.end_elements
   end