pgsql: Provide stable test for NULL-values in KNN SP-GiST

Provide stable test for NULL-values in KNN SP-GiST

f5f084fc3e has removed test because of its instability.  This commit provides
alternative test with determined ordering using extra ORDER BY expression.

Backpatch-through: 12

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/5033e9580869fec514d787dc9d3b0b63cce0bcfb

Modified Files
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src/test/regress/expected/create_index_spgist.out | 10 ++++++++++
src/test/regress/sql/create_index_spgist.sql      |  4 ++++
2 files changed, 14 insertions(+)