Fix bugs in FileSystemObject text-file handling
OpenTextFile(...,ForWriting,True) should either create a new file, or open and truncate an existing one
OpenTextFile(...,ForAppending,?,True) should write a BOM if appending to an existing-but-empty file
-- v5: scrrun: Fix FileSystemObject.OpenTextFile when writing to existing files. scrrun/tests: Introduce a test_file_contents helper.