Fix test_undecodable_env of test_subproces for non-ASCII directory

This test was introduced by r80421 (issue #8391).

The fix: copy the environment variables instead of starting Python in an empty
environement. In an empty environment, the locale is C and Python uses ASCII
for the default file system encoding. The non-ASCII directory will be encoded
using surrogates, but Python3 is unable to load a module or package with a
filename using surrogates.

See issue #8242 for more information about running Python3 with a non-ascii
directory in an empty environement.

Lib/test/test_subprocess.py

@@ -807,9 +807,11 @@ class POSIXProcessTestCase(BaseTestCase):
 
             # test str with surrogates
             script = "import os; print(repr(os.getenv(%s)))" % repr(key)
+            env = os.environ.copy()
+            env[key] = value
             stdout = subprocess.check_output(
                 [sys.executable, "-c", script],
-                env={key: value})
+                env=env)
             stdout = stdout.rstrip(b'\n\r')
             self.assertEquals(stdout, value_repr)
 
@@ -817,9 +819,11 @@ class POSIXProcessTestCase(BaseTestCase):
             key = key.encode("ascii", "surrogateescape")
             value = value.encode("ascii", "surrogateescape")
             script = "import os; print(repr(os.getenv(%s)))" % repr(key)
+            env = os.environ.copy()
+            env[key] = value
             stdout = subprocess.check_output(
                 [sys.executable, "-c", script],
-                env={key: value})
+                env=env)
             stdout = stdout.rstrip(b'\n\r')
             self.assertEquals(stdout, value_repr)