test: simplify pyc logic

The old pycache_prefix logic was because we would run tests in Docker
containers, and they went faster if we set PYTHONCACHEPREFIX.  But we
don't do that anymore, so we can use simpler code.

tests/test_execfile.py

@@ -8,6 +8,7 @@
 import json
 import os
 import os.path
+import pathlib
 import re
 import sys
 
@@ -121,15 +122,7 @@ def doit():
         os.remove("compiled.py")
 
         # Find the .pyc file!
-        roots = ["."]
-        prefix = getattr(sys, "pycache_prefix", None)
-        if prefix:
-            roots.append(prefix)
-        for root in roots:                              # pragma: part covered
-            for there, _, files in os.walk(root):       # pragma: part covered
-                for fname in files:
-                    if fnmatch.fnmatch(fname, "compiled*.pyc"):
-                        return os.path.join(there, fname)
+        return str(next(pathlib.Path(".").rglob("compiled*.pyc")))
 
     def test_running_pyc(self):
         pycfile = self.make_pyc()