fixup test directory

C++ tests load from /data/test/ again, and the _main includes in
data/ and data/test/ are fixed up to accomodate this.

This fixes up cc138a4ed96f3ac1f8c5130aa6842d1429013da4
fixup run_wml_tests

Prev version could sometimes give an error if test timed out and
error.log was not created, so add a check for if error.log exists.

Also refactor so the error log handling is in its own function.
Merge branch 'master' of git://github.com/wesnoth/wesnoth