[follow-up #3846] Restructure pool-tests

[Ben-PH]

#3486 re-instates some pool tests, but there are some refinements that would be nice to add:

• re-use of the mock provider function for get_block_operations
• use of pool_test function in the test_pool test
• re-use of the thread setup in test_pool test
• Clear less than helpful renames
• confirm no remaining ignored tests

[Ben-PH]

Duplicated code segments I've found:

massa/massa-protocol-exports/src/tests/tools.rs

Line 179 in 61f7bb9

pub fn create_operation_with_expire_period(

massa/massa-pool-worker/src/tests/tools.rs

Line 26 in 61f7bb9

pub fn create_operation_with_expire_period(

...if you follow where each of these methods are called from, protocol impl has maaaaaany call locations. Could look at them to see how things can be DRY'd up