Commit 9d701e62 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