pgsql: Improve coverage of psql for backslash commands with \if and\el

Improve coverage of psql for backslash commands with \if and \elif

This adds tests to cover more code paths to ignore backslash commands in
false branches when using \if|\elif|\else, and improves the coverage of
\elif.

Author: Fabien Coelho
Discussion: https://postgr.es/m/alpine.DEB.2.21.1908281618520.28828@lancre

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aafe2762b152ffd4cb839d2a01df6cfcc3c6df6c

Modified Files
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src/test/regress/expected/psql.out | 63 +++++++++++++++++++++++++++++++++-----
src/test/regress/sql/psql.sql      | 62 ++++++++++++++++++++++++++++++++-----
2 files changed, 111 insertions(+), 14 deletions(-)