make test assertions relative in IsolatedPluginTests

relates to #5621
[TEST] skip explain reroute REST test until param substitution is implemented
Clarify range aggregations
Remove same-thread checks when recycling (see #5254).
Prevent page-tracking checks from spreading across tests.
Reuse pages more agressively in BigArrays.

Pages are now going to be reused when the size of the array is greater than
${page_size}/2 (as opposed to ${page_size} currently).

Close #5299