slt test coverage for CASE exprs with constant value lookup tables

[alamb]

Which issue does this PR close?

• follow on to perf: optimize CASE WHEN lookup table (2.5-22.5 times faster) #18183 from @rluvaton and @pepijnve

Rationale for this change

I want to ensure we have test coverage for constant value lookup tables in the SLT tests

What changes are included in this PR?

1. Add some more slt tests that use constant value lookup tables (e.g. when the case values are all constants)

I also ran

cargo llvm-cov test --html  --test sqllogictests -- case

To ensure this was covering the new code

Are these changes tested?

Only tests

Are there any user-facing changes?

NO

[alamb]

Thanks @rluvaton