Fix benchmark usage, add instructions

.github/instructions/unit-test.instructions.md

@@ -0,0 +1,8 @@
+---
+applyTo: '**/*.test.ts'
+---
+When writing unit tests follow these rules:
+
+- When writing unit tests for addons, always create a real xterm.js instance instead of mocking it.
+- Prefer `assert.ok` over `assert.notStrictEqual` when checking something is undefined or not.
+- Avoid comments as most tests should be self-documenting.