pgsql: Add some test cases to improve test coverage of parse_expr.c.

Add some test cases to improve test coverage of parse_expr.c.

I chanced to notice while thumbing through lcov reports that we had
exactly no coverage of BETWEEN SYMMETRIC, nor of current_time(N) and
localtime(N). Improve that.

parse_expr.c still has a pretty awful coverage number, but a large part
of that is due to lack of coverage of the operator_precedence_warning
logic. I have zero desire to write tests for that; I think ripping it
out would be more sensible at this point.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/cf20cc00a99155a8e41a1bb2a1e498624c86db29

Modified Files
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src/test/regress/expected/expressions.out | 85 ++++++++++++++++++++++++++++++-
src/test/regress/sql/expressions.sql | 33 +++++++++++-
2 files changed, 116 insertions(+), 2 deletions(-)