Use double-underscore instead of non-ASCII character for class name separator

Use double-underscore instead of non-ASCII character for class name separator

MediaWikiParserTest.php generates fake test classes with eval(). It uses
synthetic class names with U+2044 "fraction slash" as a separator, but
this turns out to be an unfortunate choice since in certain terminal
modes, it causes readline to return to the start of the line as if the
"home" key was pressed, without adding a character. This makes it
difficult to paste class names.

Change-Id: I1c66b9caf256b8d0535fb7ed6e52ed842e193f46