fixup cleanup wml test script

Also only use --validcache conservatively in the -u mode.

Further fixup some comments.

This fixes up commit 5b87baf1e46939205454b803ef7b2525d494432d
wml test script can recieve list of tests as an argument

Also, fixup documentation.
better handling of error logs for wml unit test script

error logs get pooled in errors.log
This is necessary because we don't stop testing when we get an
error anymore.