add margin of error to the test

Sometimes this is flaky on CI, even though it doesn't make any sense
because we use `setTimeout(_, 500)` which should _at least_ result in
500ms...

But sometimes this results into:
```
AssertionError: expected 499.76 to be greater than or equal to 500
```

packages/@tailwindcss-node/src/instrumentation.test.ts

@@ -83,7 +83,7 @@ it('should measure async callbacks via the `span` api', async () => {
     `)
 
     let [, duration] = stripVTControlCharacters(output).match(/\[(.*)\]/)!
-    expect(parseFloat(duration)).toBeGreaterThanOrEqual(500)
+    expect(parseFloat(duration)).toBeGreaterThanOrEqual(500 - 2 /* Margin of error */)
   })
 })