update test_cd with pushd/popd

test/test_cd.ipy

@@ -1,6 +1,18 b''
+# assumes there is /tmp and /opt
+
+def curpath():
+    cwd = os.path.splitdrive(os.getcwd())[1].replace('\\','/')
+    print cwd
+    return cwd
+
 import os
 cd /
-assert os.getcwd() == '/'
+assert curpath() == '/'
 %cd /tmp
+assert curpath() == '/tmp'
+pushd /opt
+assert curpath() == '/opt'
+popd
+assert curpath() == '/tmp'
+
 
-assert os.getcwd() == '/tmp'