atests: Found two more html resource files and converted them to .robot (#1766)
atest: Removed invalid_syntax.robot/html suite because it contained duplicate tests.

Initially planned to change test data file to use .robot extension
instead of .html (#1766). After looking at the file noticed that most
tests were duplicated with tests elsewhere. Those that were not dupes
were moved elsewhere and so the suite could be removed altogerher.
test fix after recent atest cleanup
genrunner: Use atest_resource.robot, not txt. Also check input path.