Extract all functions and objects used in tests into functions in the `R/` directory or into testthat helpers

[krlmlr]

Rationale: Make each test_that() block run independently without having to run setup code.

Example: tarjan_yannakakis_graph

[maelle]

But load_all() loads helper files? And why have code useful only for tests in R/?

[krlmlr]

Moving from helpers to R/ is a separate discussion. This is just about not having setup code sit loosely in test files.