fix(test): ignore explain memo cost

[dqhl76]

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Summary

• fixes: bug: flaky test on memo mix_property.test #19230
• caused by: fix: use full join keys for hash shuffle to avoid skew #19198

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Not run (CI will cover sqllogic)

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is