Fix testifylint violations in fail_on_violation_test.go

[Copilot]

Changes

• require-error: Replace assert.EqualError with require.EqualError for the error message assertion
• error-is-as: Replace assert.True(t, errors.Is(...)) with assert.ErrorIs(...) directly
• Remove now-unused "errors" import

// Before
assert.EqualError(t, ErrViolationsFound, "poutine: violations found")
assert.True(t, errors.Is(ErrViolationsFound, ErrViolationsFound))

// After
require.EqualError(t, ErrViolationsFound, "poutine: violations found")
assert.ErrorIs(t, ErrViolationsFound, ErrViolationsFound)