Consider removing the `$start` argument from `WP_UnitTest_Generator_Sequence`

[Frank Klein]

As a follow up to [35244] and [37299], I wonder whether the $start argument in WP_UnitTest_Generator_Sequence still makes sense.

Currently the class handles two things:

• Global increments (via the static incrementor variable).
• Local increments (via the $start argument).

So I'd propose to remove the $start argument. A different, simpler class could be used for local increments instead.