Add Test Function for dummy.clear

test/test_function_dummy_clear.py

@@ -0,0 +1,8 @@
+from nose.tools import assert_raises
+from escpos.printer import Dummy
+
+def test_printer_dummy_clear():
+    printer = Dummy()
+    printer.text("Hello")
+    printer.clear()
+    assert(printer.output == b'')