Allow sections in test DataReader to appear in any order

[abroglesc]

From a comment by @bxparks in #57 regarding the sections within the DataReader class.

Hmm, it's getting harder to keep track of which tags are allowed in which sections. Originally, the order of the tags were just: DATA, [ERRORS], SCHEMA, END. But now it's DATA, [EXISTING_SCHEMA], [ERRORS], SCHEMA, END. A better way would be to allow these sections to appears in any order. But that's a bit out of scope for this PR. If I get motivated, maybe I'll take a crack at it after merging in this PR... but realistically, it will probably not rise high enough on my priority list with so many other things going on. Too bad. At least this tidbit is recorded here.