Minor: refine memory_limit tests #2117

Job	Run time
Verify Vendored Code	58s
cargo test (mac)	20m 47s
Check cargo fmt	41s
cargo check	9m 16s
Verify MSRV	10m 22s
cargo test (win64)	16m 42s
cargo test doc (amd64)	18m 45s
cargo test (amd64)	12m 54s
cargo doc	5m 5s
cargo test datafusion-cli (amd64)	5m 49s
Run sqllogictest with Postgres runner	5m 1s
cargo examples (amd64)	10m 31s
verify benchmark results (amd64)	43m 10s
check Cargo.toml formatting	1m 50s
clippy	8m 1s
check configs.md is up-to-date	4m 57s
cargo test hash collisions (amd64)	14m 29s
cargo test pyarrow (amd64)	2m 35s
	3h 11m 53s

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