Challenge: Simulate a Transfer Test in Rust

Test-driven development is critical for Solana programs. You'll write a function that simulates a complete transfer test scenario — the same logic you'd verify with anchor test.

Your Task

Implement simulate_transfer_test(sender_initial, transfer_amount, fee) that:

1. Checks if sender can afford transfer_amount + fee
2. If yes, computes final balances and returns "PASS:sender=<final>,receiver=<final>"
3. If no, returns "FAIL:insufficient_funds"

Test constants:

• Receiver starts with 0 balance
• Fee is paid by sender and burned (not added to receiver)
• The function simulates what an Anchor test's assert checks would verify

This mirrors the Anchor test pattern:

it('transfers SOL correctly', async () => {
  const balanceBefore = await connection.getBalance(sender);
  await program.methods.transfer(amount).rpc();
  const balanceAfter = await connection.getBalance(sender);
  expect(balanceAfter).to.equal(balanceBefore - amount - fee);
});