string: Convert KUnit test names to standard convention

The KUnit convention for test names is AREA_test_WHAT. Adjust the string
test names to follow this pattern.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Tested-by: Ivan Orlov <ivan.orlov0322@gmail.com>
Link: https://lore.kernel.org/r/20240419140155.3028912-5-keescook@chromium.org


Signed-off-by: Kees Cook <keescook@chromium.org>