[xharness] Fix rendering human-readable results for parameterized tes…

…ts. Fixes #9400. (#9704)

We weren't probably parsing nested test-suite elements in the xml, so
implement parsing of test-suite elements using recursion, which is easier and
less error-prone (and turns out to work fine too).

Also use TextReader/Writer instead of StreamReader/Writer in a few places.
This makes it easier to write tests, since TextReader/Writer can use in-memory
streams (no need to mess with temporary files).

Fixes #9400.