stacks-node: evaluate prefix usage for test naming

[fdefelici]

Currently no standard naming of test functions exists.

• Some just start with the test scenario (e.g. "do_check_this_thing")
• Other start with "test_" suffix (e.g. "test_do_check_this_thing")

The idea is to prefix test based on the type:

• for unit tests prefix "ut_" (e.g. "ut_do_unittest_things")
• for integration tests prefix "it_" (e.g. "it_do_intetest_things")

(Or eventually, just use the "test_" prefix for all the test functions)

This, not just for test filtering (because it will be possible using proper cargo test flag), but mainly to simplify function recognition type recognition in the IDE (using the outline for instance) and disambiguate easily from utility functions and test functions.

We could start in stacks-node to roll-out the standard and then spread incrementally in other packages later on