run-tests: move t test parsing into its own function
Test parsing is somewhat complicated. This patch extracts it into its
own function.
The impetus of this patch is folding tsttest() into the TTest class.
Subsequent patches will continue this work until tsttest() no longer
exists.
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null