test: remove copy-pasted tip() function when unused

Summary:
Remove the tip() function in abc_ tests when it is unused or used only once (set the attribute directly).

When the function is used multiple times in a test, make it a method and rename it to set_tip().

Test Plan: test/functional/test_runner.py abc*

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D10297