Restricts visibility to tests #170

Job	Run time
Check License Header	2m 29s
cargo check	7m 54s
build with wasm-pack	2m 48s
cargo test (macos)	15m 18s
cargo test (macos-aarch64)	15m 14s
Verify MSRV (Min Supported Rust Version)	8m 56s
Check cargo fmt	42s
Verify Vendored Code	52s
cargo test (win64)	1h 8m 6s
clippy	6m 31s
cargo test hash collisions (amd64)	17m 47s
check Cargo.toml formatting	2m 56s
check configs.md and ***_functions.md is up-to-date	3m 15s
cargo doc	4m 30s
cargo test (amd64)	11m 40s
cargo test datafusion-cli (amd64)	4m 5s
cargo test doc (amd64)	16m 32s
Run sqllogictest with Postgres runner	3m 48s
cargo examples (amd64)	13m 57s
cargo test pyarrow (amd64)	3m 5s
verify benchmark results (amd64)	13m 18s
	3h 43m 43s

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