Add percent sign assertion to tests

Author: CRQuadro

tests/test_05_tools/test_514_text.py

@@ -210,6 +210,7 @@ def test_plain_text():
     assert plain_text("%%C") == "Ø"  # alt-0216
     assert plain_text("%%D") == "°"  # alt-0176
     assert plain_text("%%P") == "±"  # alt-0177
+    assert plain_text("%%%") == "%"  # literal percent sign
     # underline
     assert plain_text("%%u") == ""
     assert plain_text("%%utext%%u") == "text"