Switch amount measurement from floating point to integers

[IsaacCheng9]

Summary

• The unit test for settling debt is currently non-deterministic, likely due to floating point arithmetic issues
  • Avoid floating point arithmetic by measuring currency amounts in integers instead of floating points
  • Switch the unit from pounds (GBP) to pence (GBX)
• See example of non-deterministic test runs here (screenshot below):

Test Code

  // Check whether we can settle a debt between two users.
  test("POST /debts/settle", async () => {
    const server = supertest(app);
    await server
      .post("/debts/settle")
      .send({
        from: "testuser456",
        to: "testuser123",
        amount: 50,
      })
      .expect(200);

    // Check whether the debt was successfully reduced by 50.
    await debtModel
      .findOne({
        from: "testuser123",
        to: "testuser456",
      })
      .then((debt) => {
        expect(debt.amount).toBe(50);
      });
  });