Small improvements to test-one.sh
Small improvements to test-one.sh

Allow running tests that don't have `bindgen-flags` pragmas. Less noisy
output. Show diff if the generated bindings differ from the expected bindings.

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