Remove unused imports in tests for yaml_load.rs

[rhysd]

While working on #530, I noticed cargo test warned unused imports in test code.

warning: unused import: `crate::parsing::syntax_definition::*`
   --> src/parsing/yaml_load.rs:890:9
    |
890 |     use crate::parsing::syntax_definition::*;
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[warn(unused_imports)]` on by default

This PR fixes the warning.