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@gmail.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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