gpython: tests: Don't assume path delimiter is /

Because on Windows it is \ and using / for it to scan in path fails and
leads to test_pymain_opt failure:

     E             Full diff:
     E               [
     E                'sys.flags.debug:      0',
     E                'sys.flags.optimize:   0',
     E                '__debug__:            True',
     E                'assert:               True',
     E                'docstrings:           True',
     E                'import mod.py:        '
     E             -  'C:\\users\\kirr\\Temp\\modpy_imports_fromvmgs8go4\\__pycache__/mod.cpython-310.pyc',
     E             ?                                            ^^  ----
     E             +  'C:\\users\\kirr\\Temp\\modpy_imports_from850gb81s\\__pycache__/mod.cpython-310.pyc',
     E             ?                                            ^^^ +++
     E               ]

Here the difference is because gpython/testprog/print_opt.py failed to
detect tmpd prefix and strip it.

gpython/testprog/print_opt.py

 # -*- coding: utf-8 -*-
-# Copyright (C) 2020  Nexedi SA and Contributors.
+# Copyright (C) 2020-2023  Nexedi SA and Contributors.
 #                          Kirill Smelkov <kirr@nexedi.com>
 #
 # This program is free software: you can Use, Study, Modify and Redistribute
@@ -62,8 +62,9 @@ def modpy_imports_from():
         raise AssertionError("module 'mod' is already there")
 
     tmpd  = tempfile.mkdtemp('', 'modpy_imports_from')
+    tmpd_ = tmpd + os.path.sep
     try:
-        pymod = "%s/mod.py" % tmpd
+        pymod = tmpd_ + "mod.py"
         with open(pymod, "w") as f:
             f.write("# hello up there\n")
 
@@ -73,9 +74,9 @@ def modpy_imports_from():
         files = set()
         for dirpath, dirnames, filenames in os.walk(tmpd):
             for _ in filenames:
-                f = '%s/%s' % (dirpath, _)
-                if f.startswith(tmpd+'/'):
-                    f = f[len(tmpd+'/'):]
+                f = os.path.join(dirpath, _)
+                if f.startswith(tmpd_):
+                    f = f[len(tmpd_):]
                 files.add(f)