fix custom middleware pipeline builders for testsuite

fix custom middleware pipeline builders for testsuite
commit_hash:e078e8f7f481706b90641f23338f745abb57bcfa

Author: joker2k

testsuite/pytest_plugins/pytest_userver/plugins/config.py

@@ -750,10 +750,10 @@ def patch_config(config_yaml, config_vars):
         if 'server' not in components:
             return
 
-        pipeline_builder = components.setdefault(
-            'default-server-middleware-pipeline-builder',
-            {},
+        pipeline_builder_name = components['server'].get(
+            'middleware-pipeline-builder', 'default-server-middleware-pipeline-builder'
         )
+        pipeline_builder = components.setdefault(pipeline_builder_name, {})
         middlewares = pipeline_builder.setdefault('append', [])
         middlewares.append('testsuite-exceptions-handling-middleware')