GC test polishing

[jmid]

Analogous to #558 I also had another look at the Gc tests and spotted a couple of things there.
This PR is the result.

• The biggest change is presentational, where the generator pair is split and instead defined one after another.
• Second it replaces minor_heap_size_gen with a bit-shifting version rather than "choose from list literal".
• Third it generates Counters as part of alloc_cmds (like the other observational Gc functions) for consistency.

Finally there are a few smaller meaning preserving code clean ups.

[jmid]

CI summary: all 50 workflows passed

[jmid]

CI summary for merge to main: all 51 workflows passed