Fix portability issue in authentication test 003_peer

Fix portability issue in authentication test 003_peer

The mapped user name is built upon the OS user name of the environment
where the test is run.  Depending on the characters used in the OS user
name, CREATE ROLE may not get parsed (the author has mentioned hyphens
as one case), causing a failure of the test.

Let's use double-quotes around the mapped user name, which should be a
solution good enough for the environments where this test tends to run.
The buildfarm issued no complaint over the years.

Oversight in 3c4e26a62c31, so backpatch down to v19.  Perhaps
3c4e26a62c31 and this commit should be backpatched further down, but
let's leave that for another day, if it proves necessary.

Author: Yugo Nagata <nagata@sraoss.co.jp>
Discussion: https://postgr.es/m/20260727133857.fbd23d43d422f10f376a8bee@sraoss.co.jp
Backpatch-through: 19