Minor: include the number of files run in sqllogictest display

[alamb]

Which issue does this PR close?

• Follow on to Restore ability to run single SLT file #14355
• Follow on to Add sqlite test files, progress bar, and automatic postgres container management into sqllogictests #13936

Rationale for this change

While testing #14355 from @findepi I found it hard to see how many files were run

What changes are included in this PR?

Before this PR:

andrewlamb@Andrews-MacBook-Pro-2:~/Software/datafusion$ cargo test --test sqllogictests  -- union
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.16s
     Running bin/sqllogictests.rs (target/debug/deps/sqllogictests-3d953efd822e1e41)
Completed in 0 seconds

After this PR, it reports Completed 2 tests:

andrewlamb@Andrews-MacBook-Pro-2:~/Software/datafusion$ cargo test --test sqllogictests  -- union
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.16s
     Running bin/sqllogictests.rs (target/debug/deps/sqllogictests-3d953efd822e1e41)
Completed 2 tests in 0 seconds

Are these changes tested?

I tested them manually

Are there any user-facing changes?

[alamb]

This function was already basically returning a Vec so there didn't seem to be any need to have the iterator indirection

[alamb]

Thanks again @findepi