Add //ignore-macos to pretty-std-collections.rs
Add //ignore-macos to pretty-std-collections.rs

On macOS the test is flaky and sometimes fails,
sometimes succeeds on CI.

graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust