Unit tests for FastAPI branch

Run via `make test`

tests/helpers.py

@@ -0,0 +1,19 @@
+import hashlib
+import secrets
+from lnbits.core.crud import create_payment
+
+async def credit_wallet(wallet_id: str, amount: int):
+    preimage = secrets.token_hex(32)
+    m = hashlib.sha256()
+    m.update(f"{preimage}".encode())
+    payment_hash = m.hexdigest()
+    await create_payment(
+        wallet_id=wallet_id,
+        payment_request="",
+        payment_hash=payment_hash,
+        checking_id=payment_hash,
+        preimage=preimage,
+        memo="",
+        amount=amount,# msat
+        pending=False,# not pending, so it will increase the wallet's balance
+    )