test: sample fixture course

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

fixtures/courses/sample/exercises/add/solution.js

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+export function add(a, b) {
+  return a + b;
+}

fixtures/courses/sample/exercises/add/starter.js

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+export function add(a, b) {
+  return a - b;
+}

fixtures/courses/sample/exercises/add/tests.js

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+export const tests = [
+  { name: "adds 2 and 3", run: (mod, assert) => assert.equal(mod.add(2, 3), 5) },
+  { name: "adds -1 and 1", run: (mod, assert) => assert.equal(mod.add(-1, 1), 0) },
+];