refactor(test): improve planner test suite for case when & add const evaluation test

[xzhseh]

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What's changed and what's your intention?

As titled, should merge after #14965.

Note that the base branch is set to #14965 just to avoid any potential merge conflict. 😄

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[xzhseh]

What about merging this PR, so that #15083 could also be reviewed & merged, cc @st1page @chenzl25.