pgsql: Add a couple more tests for interval input decoding.

Add a couple more tests for interval input decoding.

Cover some cases that would have been broken by a proposed patch,
but we failed to notice for lack of test coverage.  I'm pushing
this separately mainly to memorialize that it *is* our historical
behavior.

Discussion: https://postgr.es/m/1344498.1648920056@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1b208ebaf14e668a24c78ee42cf805431cd0f591

Modified Files
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src/test/regress/expected/interval.out | 20 ++++++++++++++++++++
src/test/regress/sql/interval.sql      | 12 ++++++++++++
2 files changed, 32 insertions(+)