Test macos alias behavior

[niklasmohrin]

In #215, we added support for the "macos" platform which works as an alias for "osx". This was later included in version 2.0 of the client specification. However, we don't really have tests for this, as "osx" does not appear in our test suite at all and "macos" only appears once in another context.

Tests should be added that ensure that both "macos" and "osx" map to "osx" directory in the page cache. To do so, a test in tests/lib.rs should create a cache with a page for the "osx" platform and check that it is found in a normal tldr invocation and in tldr --list.