Add failing cross platform test for #373

[konstin]

See #373 (comment).

This test fails on 32-bit, where the iteration order of an FxHashMap is different:

$ cargo test -p pubgrub --test tests same_result_across_platforms --target i686-unknown-linux-gnu -q

running 1 test
same_result_across_platforms --- FAILED

failures:

---- same_result_across_platforms stdout ----

thread 'same_result_across_platforms' panicked at tests/tests.rs:137:5:
assertion `left == right` failed
  left: "964"
 right: "712"
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

failures:
    same_result_across_platforms

test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured; 3 filtered out; finished in 0.29s

error: test failed, to rerun pass `-p pubgrub --test tests`