Use "zero-width space" in long testing strings

Various tests create multi-megabyte documents to test how the server reacts to large documents or large quantities of data. Replacing these documents with large strings of "zero-width spaces" rather than visible characters like "aaaaaa" or ",,,,,", prefixed by "INVISIBLE_LONG_STRING" to make readers aware, would still test the expected server behavior without overwhelming terminals, editors, and browsers that display the logs.