Add mimetools testcase.

Lib/test/output/test_mimetools

+test_mimetools
+7bit PASS
+8bit PASS
+base64 PASS
+quoted-printable PASS

Lib/test/test_mimetools.py

+from test_support import TestFailed
+import mimetools
+
+import string,StringIO
+start = string.letters + "=" + string.digits + "\n"
+for enc in ['7bit','8bit','base64','quoted-printable']:
+    print enc,
+    i = StringIO.StringIO(start)
+    o = StringIO.StringIO()
+    mimetools.encode(i,o,enc)
+    i = StringIO.StringIO(o.getvalue())
+    o = StringIO.StringIO()
+    mimetools.decode(i,o,enc)
+    if o.getvalue()==start:
+        print "PASS"
+    else:
+        print "FAIL"
+        print o.getvalue()