Tests: Revert change to fixture_code

In commit a68e1e1 the `fixture_code` fixture
was adapted to query for the first `InstalledCode` with the test code label
instead of using `load_code`, as the latter would fail when running the new
parametrized PDOS tests in parallel.

However, running tests in parallel versus the same storage backend can
potentially lead to all sorts of problems. So instead of providing a somewhat
hacky fix for this case, we revert the changes made to `fixture_code` and avoid
running the tests in parallel.

tests/conftest.py

@@ -1,5 +1,5 @@
 # -*- coding: utf-8 -*-
-# pylint: disable=redefined-outer-name,too-many-statements,unsubscriptable-object
+# pylint: disable=redefined-outer-name,too-many-statements
 """Initialise a text database and profile for pytest."""
 from collections.abc import Mapping
 import io
@@ -51,17 +51,14 @@ def fixture_code(fixture_localhost):
     """Return an ``InstalledCode`` instance configured to run calculations of given entry point on localhost."""
 
     def _fixture_code(entry_point_name):
-        from aiida.orm import InstalledCode, QueryBuilder
+        from aiida.common import exceptions
+        from aiida.orm import InstalledCode, load_code
 
         label = f'test.{entry_point_name}'
 
-        query = QueryBuilder().append(
-            InstalledCode,
-            filters={'label': label},
-        )
         try:
-            return query.first()[0]
-        except TypeError:
+            return load_code(label=label)
+        except exceptions.NotExistent:
             return InstalledCode(
                 label=label,
                 computer=fixture_localhost,