Implement parameter for cargo xtask web-tests to refresh slide list to test in the web tests

[michael-kerscher]

with #2743 implementing a cargo xtask web-tests command there is opportunity for a parameter of this command to make life easier.

There is a script create-slide.list.sh that is used to create the list of available slides on the system.

It requires either the TEST_BOOK_DIR env variable (defined in the CI script) or the directory as the first parameter (wdio.conf.ts defaults to ../book/html if the environment is not set).

That could be a parameter of this command to run the script. Something along the lines of "--refresh-slide-list" or shorter.