Set correct backend for verdi quicksetup test (#2954)

The backend was set explicitly, as it should, but it used by accident the wrong
key, causing the command to silently use the default. This can lead to all sorts
of mysterious problems if the testing backend is not the same as the default.

aiida/backends/tests/cmdline/commands/test_setup.py

@@ -83,7 +83,7 @@ def test_quicksetup_from_config_file(self):
 first_name: Leopold
 last_name: Talirz
 institution: EPFL
-backend: {}
+db_backend: {}
 email: [email protected]""".format(self.backend))
             handle.flush()
             result = self.cli_runner.invoke(cmd_setup.quicksetup, ['--config', os.path.realpath(handle.name)])