Fix mutated default argument in roundtrip tests (#8179)

The test appends to `opts` which can refer to the default value of the
function, causing future calls with the default value to be changed.
e.g.

```py
**>>> def f(a=[]):
...     a.append(1)
...     print(a)
...
>>> f()
[1]
>>> f()
[1, 1]
```
This would cause `-g` to be added to the arguments even when
`debug=False`.

There are other instances that use lists as a default argument, but I
left them alone for now since they never mutate it.

test/unit/utils.py

diff --git a/test/unit/utils.py b/test/unit/utils.py
index 6fe2d4f..b2267fc 100644
--- a/test/unit/utils.py
+++ b/test/unit/utils.py
@@ -9,7 +9,8 @@
         return os.path.join(shared.options.binaryen_test, 'unit', 'input',
                             filename)
 
-    def roundtrip(self, filename, opts=[], debug=True):
+    def roundtrip(self, filename, opts=None, debug=True):
+        opts = opts if opts is not None else []
         opts = ['--mvp-features'] + opts
         if debug:
             opts.append('-g')