Fix 'Too many open files' on fuzz test.

[dhegberg]

Which issue does this PR close?

Closes #10674.

Rationale for this change

• On MacOS a batch size of 1,000,000 with a 10KB pool size results in a panic Too many open files.
• The spill threshold for 10KB pool size already spills at the 20,000 batch size, so going to 1,000,000 for this test seem unnecessary.

What changes are included in this PR?

• change upper spill test threshold batch size to 500,000 from 1,000,000.
• rename test to test_sort_10k_mem to reflect the 10KB pool size to be consistent with the test test_sort_100k_mem which uses a 100KB pool size.

Are these changes tested?

Changes are to tests.

Are there any user-facing changes?

No

[alamb]

THANK YOU ! I found this too many open files really annoying as well

[alamb]

Thank you @dhegberg

I ran this PR and verified that it no longer fails locally for me

cargo test --test fuzz

Thank you 🙏

[dhegberg]

THANK YOU ! I found this too many open files really annoying as well

Happy to help. Let me know if anything else is needed before merging.

[alamb]

I think it is all good -- thanks agian