Run test in parallel batches
Run test in parallel batches

Follow review suggestion to use chunks to run test in parallel.
Set default to 16 which works well even on my limited laptop,
and which should benefit better machine.

To run with a different batch size:
BINDGEN_TEST_BATCH_SIZE=32 cargo test

On my machine:
1 parallel test takes 3'53
2 parallel test takes 2'10
8 parallel test takes 2'08
32 parallel test takes 2'07

emilio/rust-bindgen,jdub/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen,jdub/rust-bindgen,jdub/rust-bindgen,jdub/rust-bindgen,rust-lang/rust-bindgen,rust-lang/rust-bindgen,emilio/rust-bindgen,emilio/rust-bindgen,rust-lang/rust-bindgen