pgsql: Convert pre-existing stats_ext tests to new style

Convert pre-existing stats_ext tests to new style

The regression tests added in commit 7300a69950 test cardinality
estimates using a function that extracts the interesting pieces
from the EXPLAIN output, instead of testing the whole plan. That
seems both easier to understand and less fragile, so this applies
the same approach to pre-existing tests of ndistinct coefficients
and functional dependencies.

Discussion: https://postgr.es/m/dfdac334-9cf2-2597-fb27-f0fb3753f435@2ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/dbb984128ebf6b4bd6f12b4579ee9103ef1ca749

Modified Files
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src/test/regress/expected/stats_ext.out | 526 ++++++++++++--------------------
src/test/regress/sql/stats_ext.sql      | 150 ++++-----
2 files changed, 257 insertions(+), 419 deletions(-)