Adjust tests to use Domain.recommended_domain_count

[jmid]

Some of our tests could benefit from taking Domain.recommended_domain_count into consideration

  Domain.recommended_domain_count : unit -> int
  The recommended maximum number of domains to be spawned.

Relevant tests include

• src/domain/domain_spawntree.ml
• src/domain/domain_joingraph.ml
• src/threadomain/threadomain.ml
• ...