KateDocument: always add space after single line comment start marker

So that if you have code: if (foo) { do_something(); }

commenting out the second line, it would become (assuming the position="afterwhitespace" option is set in c.xml): if (foo) { // do_something(); }

This works better with clang-format, which insists on reformatting this: //do_something(); to // do_something();

to the version with the space, "// ".

Fix unittests.

CCBUG: 456819