pgsql: Adjust EXPLAIN test case to filter out "Actual Rows" values.

Adjust EXPLAIN test case to filter out "Actual Rows" values.

Per the buildfarm, these tests appear to be unstable in the wake of
commit ddb17e387aa28d61521227377b00f997756b8a27. I'm not sure that
just hiding this output is the right way forward, because I think
there may be other test cases that will fail with lower probability
even after this fix. However, it's hard to tell right now, because
this is failing on a number of buildfarm animals. So let's try this
for now to either get a clearer picture of what else is broken, or
as a stopgap until we decide what the permanent fix should be, or
perhaps this will be the permanent fix after all.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/44cbba9a7f51a3888d5087fc94b23614ba2b81f2

Modified Files
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src/test/regress/expected/explain.out | 5 +++--
src/test/regress/sql/explain.sql      | 3 +++
2 files changed, 6 insertions(+), 2 deletions(-)