pgsql: Fix explain regression test failure.

Fix explain regression test failure.

Commit 9d701e624f caused the regression test for EXPLAIN to fail on
the buildfarm member prion. This happened because of instability of
test output, i.e., in text format, whether "Planning:" line is output
varies depending on the system state.

This commit updated the regression test so that it ignores that
"Planning:" line to produce more stable test output and get rid of
the test failure.

Back-patch to v13.

Author: Fujii Masao
Discussion: https://postgr.es/m/1803897.1598021621@sss.pgh.pa.us

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bc2ebf3acfd287247cd9ee286c44e969af1c81de

Modified Files
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src/test/regress/expected/explain.out | 3 +++
src/test/regress/sql/explain.sql      | 3 +++
2 files changed, 6 insertions(+)