fix: create target directory in fuzz targets

[pocopepe]

Closes #60

Fuzz targets wrote to target/ without creating it first, causing a NotFound panic at runtime in CI.
Also fixes a typo in fuzzing.yml where matrix.test-target.name.should_pass (always null) made continue-on-error: true for every target, silently swallowing fuzz failures since the workflow was first added.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.00%. Comparing base (a340afb) to head (b1d91ba).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@     Coverage Diff     @@
##   main   #219   +/-   ##
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