Improve SEMI JOIN testing and behaviors

[theory]

Follow up the work in #58 to ensure the quality of SEMI JOIN pushdown:

• Add tests fo edge cases (multiple EXISTS, complex correlations, etc.) and fix any issues
• Only simple Var = Var equalities extracted as join keys. Complex expressions like l_orderkey = o_orderkey + 1 won't be recognized. Add tests and fix them.
• Multiple EXISTS should work via chained LEFT SEMI JOINs; add more tests to validate.