Improve frame:callParserFunction argument validation

From wikitext, $parser->callParserFunction() will always get an array of
strings with at least an element [0]. Let's match this from Scribunto:
stringify numbers, and require that [0] (although in Lua it'll be [1]).

Also fix an old broken unit test.

Bug: 63597
Change-Id: Ie7ac34ae4bce70cec455d90c3f02a658644f6866