add test help fn

[asukaminato0721]

This pull request includes changes to improve the test code and simplify data handling. The changes focus on replacing vec![] with array literals, introducing a helper function for approximate equality assertions, and simplifying the construction of FSRSReview instances.

Improvements to test code:

• src/cosine_annealing.rs: Replaced vec![] with array literals and used the new assert_approx_eq helper function for asserting approximate equality in tests. [1] [2] [3]
• src/inference.rs: Removed the local assert_approx_eq function and replaced it with the newly introduced helper function from test_helpers.rs. [1] [2]
• src/training.rs: Replaced vec![] with array literals and used the new assert_approx_eq helper function for asserting approximate equality in tests. [1] [2] [3] [4]

Simplification of data handling:

• src/convertor_tests.rs: Simplified the construction of FSRSReview instances by using tuples and into_iter().map().collect(). [1] [2]
• src/batch_shuffle.rs: Replaced vec![] with array literals in test assertions. [1] [2]

Introduction of helper functions:

• src/test_helpers.rs: Added a new assert_approx_eq helper function to assert approximate equality of arrays in tests.

[L-M-Sherlock]

Great! The code is much more clear now!