bpo-33455: Pass os.environ in test_posix::test_specify_environment. (GH-6753)

Pass os.environ's copy to new process created at test_posix:
test_specify_environment. Otherwise important variables such as
LD_LIBRARY_PATH are not set and the child process might not work at all
in an environment where such variables are required for Python to function.

Lib/test/test_posix.py

@@ -1462,7 +1462,7 @@ class TestPosixSpawn(unittest.TestCase):
         """
         pid = posix.posix_spawn(sys.executable,
                                 [sys.executable, '-c', script],
-                                {'foo': 'bar'})
+                                {**os.environ, 'foo': 'bar'})
         self.assertEqual(os.waitpid(pid, 0), (pid, 0))
         with open(envfile) as f:
             self.assertEqual(f.read(), 'bar')