Clean up Wast directives processor

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 75.43860% with 14 lines in your changes missing coverage. Please review.

Project coverage is 81.27%. Comparing base (085dbd2) to head (4a02b2e). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
crates/wasmi/tests/spec/runner.rs	75.43%	14 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1275 +/- ## ======================================= Coverage 81.26% 81.27% ======================================= Files 306 306 Lines 25311 25301 -10 ======================================= - Hits 20569 20563 -6 + Misses 4742 4738 -4 ```

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