vim.tbl_flatten is deprecated and will be removed in 0.13. Just replaced
it with the recommendation in the deprecation notice.
For some reason discovering tests did not work anymore after upgrading from
0.11.x to 0.12.1 even though vim.tbl_flatten still works in this
version. Replacing it fixes test discovery for me though and it needs to
be done before 0.13 anyway.
Co-authored-by: Sam Castelain <sam@secury-360.com>
