Don't set up YAML constructors/resolvers for default loaders

After reading https://pyyaml.org/wiki/PyYAMLDocumentation again, turns out
Loader.add_constructor and .add_implicit_resolver are actually *class* methods.

In other words, we've been adding dozens of constructors/resolvers to the
default YAML loader object, causing it to slow down massively in other tests
which call configdata.init().

Instead, create our own loader class and only add them once there.

I'm still not sure why this caused the duration to increase with every YAML load
though - that might still be some kind of bug in PyYAML.

Fixes #2777

tests/end2end/fixtures/quteprocess.py

@@ -784,13 +784,8 @@ class QuteProc(testprocess.Process):
         be compared.
         """
         __tracebackhide__ = lambda e: e.errisinstance(pytest.fail.Exception)
-        # Translate ... to ellipsis in YAML.
-        loader = yaml.SafeLoader(expected)
-        loader.add_constructor('!ellipsis', lambda loader, node: ...)
-        loader.add_implicit_resolver('!ellipsis', re.compile(r'\.\.\.'), None)
-
         data = self.get_session()
-        expected = loader.get_data()
+        expected = yaml.load(expected, Loader=YamlLoader)
         outcome = testutils.partial_compare(data, expected)
         if not outcome:
             msg = "Session comparison failed: {}".format(outcome.error)
@@ -798,6 +793,18 @@ class QuteProc(testprocess.Process):
             pytest.fail(msg)
 
 
+class YamlLoader(yaml.SafeLoader):
+
+    """Custom YAML loader used in compare_session."""
+
+    pass
+
+
+# Translate ... to ellipsis in YAML.
+YamlLoader.add_constructor('!ellipsis', lambda loader, node: ...)
+YamlLoader.add_implicit_resolver('!ellipsis', re.compile(r'\.\.\.'), None)
+
+
 def _xpath_escape(text):
     """Escape a string to be used in an XPath expression.