Migrate memory_pool/pool tests to insta

[ding-young]

Description

In #15178, we're switching hard-coded constants in tests to insta.

This issue targets updating tests in pool.rs (datafusion/execution/memory_pool/pool) so that testing long string err messages is easier via snapshot tests.

One example that needs to be switched to insta is

datafusion/datafusion/execution/src/memory_pool/pool.rs

Line 512 in 3e30f77

assert_eq!(err, "Resources exhausted: Failed to allocate additional 30.0 B for s4 with 0.0 B already allocated for this reservation - 20.0 B remain available for the total pool");

Reference

• https://datafusion.apache.org/contributor-guide/testing.html#snapshot-testing
• PRs linked to original issue will help how to migrate these tests.

[alamb]

cc @qstommyshu and @blaginin

[lifan-ake]

take