[test] Add more tests for multiply()

[forfudan]

This pull request primarily focuses on adding comprehensive tests for the multiplication operation of the Decimal type and refactoring some related code. The most important changes include adding a new test file for multiplication, updating the multiply function to simplify its logic, and removing old multiplication tests from the test_arithmetics.mojo file.

Addition of Comprehensive Multiplication Tests:

• tests/test_multiply.mojo: Added a new file with detailed tests for the Decimal multiplication operation, covering basic multiplication, special cases, negative numbers, precision and scale handling, boundary values, and the commutative property.

Code Simplification:

• src/decimojo/arithmetics.mojo: Simplified the multiply function by directly returning a new Decimal instance for the zero case, removing the need for intermediate variables.

Removal of Redundant Tests:

• tests/test_arithmetics.mojo: Removed the old test_multiplication function and its invocation in the main function, as these tests have been replaced by the new comprehensive tests in tests/test_multiply.mojo. [1] [2]

Miscellaneous Updates:

• benches/bench.mojo: Added bench_round to the list of benchmarks and included it in the main function to ensure it is executed. [1] [2]
• mojoproject.toml: Added a new command test_multiply to run the multiplication tests.