pgsql: Adjust new pg_read_file() test cases for more portability.

Adjust new pg_read_file() test cases for more portability.

It's allowed for an installation to remove postgresql.auto.conf,
so don't rely on that being present.  Instead probe whether we can
read postmaster.pid.  (If you've removed that, you broke the data
directory's multiple-postmaster interlock, not to mention pg_ctl.)
Per gripe from Michael Paquier.

Discussion: https://postgr.es/m/YuSZTsoBMObyY+vT@paquier.xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d10fad96c664a503a4203fc4b5acd4e816b0158a

Modified Files
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src/test/regress/expected/misc_functions.out | 14 +++++++-------
src/test/regress/sql/misc_functions.sql      | 10 +++++-----
2 files changed, 12 insertions(+), 12 deletions(-)