pgsql: Add regression tests for psql's \o and \g on files

Add regression tests for psql's \o and \g on files

This adds coverage for a few scenarios not checked yet in psql, with
multiple query combined across files defined by \o, \g or both at the
same time.  The results are saved in the output files defined, then
reloaded back to check what has been written to them.

Author: Daniel Verite
Discussion: https://postgr.es/m/25c2bb5b-9012-40f8-8088-774cb764046d@manitou-mail.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d2a4490401f2ddc878134a1fd75ef4482e403e47

Modified Files
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src/test/regress/expected/psql.out | 131 +++++++++++++++++++++++++++++++++++++
src/test/regress/sql/psql.sql      |  64 ++++++++++++++++++
2 files changed, 195 insertions(+)