Add comments around testData().expirationBlock

- explicitly warn not to hard-code the value and point to
  examples/loan-cycle.ts
- explicitly mention that this function is used only for testing

Developer note: testData() is used in many places (I also think as part
of creditcoin-squid test suite) usually prior to calling beforeAll() and
thus the Creditcoin API isn't initialized yet. IMO it's too many changes
just to be able to read the last block number from a running chain.

creditcoin-js/src/testUtils/index.ts

@@ -26,6 +26,8 @@ export type TestData = {
 export const testData = (ethereumChain: Blockchain, createWalletF: CreateWalletFunc): TestData => {
     return {
         blockchain: ethereumChain,
+        // WARNING: don't hard-code this, see examples/loan-cycle.ts
+        // NOTICE: this function is only used during testing before the Creditcoin API has been initialized
         expirationBlock: 10_000_000,
         createWallet: createWalletF,
         keyring: new Keyring({ type: 'sr25519' }),