Use ".notvalid." instead of ".invalid." for files

"notvalid" is much less likely to be part of a filename in an existing
testsuite. e.g., the ARIA 1.0 test suite has some files that test the
"aria-invalid" attribute, so the filenames for those tests include the
string "invalid"...
Merge pull request #41 from sideshowbarker/notvalid

Use ".notvalid." instead of ".invalid." for files