Rename the role in stats_ext to have regress_ prefix
Commit
5be8ce82e8 added a new role to the stats_ext regression suite,
but the role name did not start with regress_ causing failures when
running with ENFORCE_REGRESSION_TEST_NAME_RESTRICTIONS. Fixed by
renaming the role to start with the expected regress_ prefix.
Backpatch-through: 10, same as the new regression test
Discussion: https://postgr.es/m/
1F238937-7CC2-4703-A1B1-
6DC225B8978A%40enterprisedb.com