Fix parsing of negated long options in wxCmdLineParser.

Negated long options were not recognized in wxCmdLineParser::Parse(), do check
for them now.

Also extend the unit test to check for negated options.

Closes #13335.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@68316 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -437,6 +437,10 @@ Major new features in this release
 2.9.3:
 ------
 
+All:
+
+- Fix parsing of negated long options in wxCmdLineParser (roed_bis).
+
 All (GUI):
 
 - Support float, double and file name values in wxGenericValidator (troelsk).