Update wvtest.py to the new version that does os.chdir().

Update wvtest.py to the new version that does os.chdir().

It was too annoying to test submodules, because if any of them opened a
file, they always had to mention the full path from the wvtest.py (which was
usually at the root of the whole project) to the data file (which was
usually in the t/ subdir of the submodule).  So if you extracted a submodule
into its own project, all the paths would be wrong.

Now wvtest.py does os.chdir() into the t/ subdir being tested.  If you don't
like that, you can always chdir into a parent directory.