test: utils: Add helpers for automatic numbering of test cases

test: utils: Add helpers for automatic numbering of test cases

Adding or reordering test cases is usually a pain due to static test
case names that are then passed to virtTestRun(). To ease the numbering
of test cases, this patch adds two simple helpers that generate the test
names according to the order they are run. The test name can be
configured via the reset function.

This will allow us to freely add test cases in middle of test groups
without the need to re-number the rest of test cases.