test: try to unflake a test

On 3.14t, this test fails occasionally:

```
>       assert any(d < 50 * 1024 for d in deltas)
  E       assert False
  E        +  where False = any(<generator object MemoryLeakTest.test_eval_codeobject_leak.<locals>.<genexpr> at 0x42d3c112300>)

  /home/runner/work/coveragepy/coveragepy/tests/test_oddball.py:242: AssertionError
  ----------------------------- Captured stdout call -----------------------------
  Mem delta: 6912
  Mem delta: 6912
  Mem delta: 7808
  Mem delta: 8916
  Mem delta: 7936
  Mem delta: 7808
  Mem delta: 7936
  Mem delta: 7808
  Mem delta: 7936
  Mem delta: 1728
```

Author: nedbat

tests/test_oddball.py

@@ -233,7 +233,7 @@ def test_eval_codeobject_leak(self, branch: bool) -> None:
         # one of our loops only increased the footprint by a small amount.
         base = osinfo.process_ram()
         deltas = []
-        for _ in range(10):
+        for _ in range(30):
             self.check_coverage(code, lines=[1, 2, 3], missing="", branch=branch)
             now = osinfo.process_ram()
             deltas.append(now - base)