BUG #15006: "make check" error if current user is "user"

The following bug has been logged on the website:

Bug reference: 15006
Logged by: Sergey Burladyan
Email address: eshkinkot(at)gmail(dot)com
PostgreSQL version: 10.1
Operating system: Debian GNU/Linux 9 (stretch) in docker
Description:

I run tests in docker with current system user "user" and "make check" stop
with error:
...
updatable_views ... ok
rolenames ... FAILED
roleattributes ... ok
...

==== /home/user/postgresql-10.1/src/test/regress/regression.diffs ====

*** /home/user/postgresql-10.1/src/test/regress/expected/rolenames.out Tue
Nov 7 00:46:52 2017
--- /home/user/postgresql-10.1/src/test/regress/results/rolenames.out Thu
Jan 11 12:51:19 2018
***************
*** 42,47 ****
--- 42,48 ----
CREATE ROLE "current_user";
CREATE ROLE "session_user";
CREATE ROLE "user";
+ ERROR: role "user" already exists
CREATE ROLE current_user; -- error
ERROR: CURRENT_USER cannot be used as a role name here
LINE 1: CREATE ROLE current_user;
***************
*** 950,952 ****
--- 951,954 ----
DROP OWNED BY regress_testrol0, "Public", "current_user",
regress_testrol1, regress_testrol2, regress_testrolx CASCADE;
DROP ROLE regress_testrol0, regress_testrol1, regress_testrol2,
regress_testrolx;
DROP ROLE "Public", "None", "current_user", "session_user", "user";
+ ERROR: current user cannot be dropped

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