BUG #15605: Unstable regression test "tablespace"

The following bug has been logged on the website:

Bug reference:      15605
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 11.1
Operating system:   Ubuntu 18.04
Description:

When running 'TESTS="$(printf "tablespace %.0s" `seq 1000`)" make
check-tests' I get:
============== running regression test queries        ==============
test tablespace                   ... ok
...
test tablespace                   ... ok
test tablespace                   ... FAILED
test tablespace                   ... FAILED
...
test tablespace                   ... FAILED
test tablespace                   ... ok
test tablespace                   ... FAILED
...
test tablespace                   ... FAILED
test tablespace                   ... ok
...
and so on...

test tablespace                   ... FAILED
============== shutting down postmaster               ==============

===========================
 403 of 1000 tests failed. 
===========================

The differences that caused some tests to fail can be viewed in the
file ".../src/test/regress/regression.diffs".  A copy of the test summary
that you see
above is saved in the file ".../src/test/regress/regression.out".

src/test/regress/regression.diffs contains:

--- .../src/test/regress/expected/tablespace.out    2019-01-23
11:40:25.438743265 +0300
+++ .../src/test/regress/results/tablespace.out     2019-01-23
11:41:00.559072671 +0300
@@ -89,9 +89,9 @@
     where c.reltablespace = t.oid AND c.relname LIKE 'part%_idx';
    relname   |     spcname
 -------------+------------------
- part1_a_idx | regress_tblspace
  part2_a_idx | regress_tblspace
  part_a_idx  | regress_tblspace
+ part1_a_idx | regress_tblspace
 (3 rows)

 -- check that default_tablespace doesn't affect ALTER TABLE index
rebuilds

======================================================================

It seems that the following query in src/test/regress/sql/tablespace.sql:
SELECT relname, spcname FROM pg_catalog.pg_tablespace t, pg_catalog.pg_class
c
    where c.reltablespace = t.oid AND c.relname LIKE 'part%_idx';
needs an "ORDER BY".