Add test for Contract.fallback_function

and `Contract.receive_function`

tests/unit/core/test_data/fallback.sol

@@ -0,0 +1,29 @@
+pragma solidity ^0.6.12;
+
+contract FakeFallback {
+    mapping(address => uint) public contributions;
+    address payable public owner;
+
+    constructor() public {
+        owner = payable(msg.sender);
+        contributions[msg.sender] = 1000 * (1 ether);
+    }
+
+    function fallback() public payable {
+        contributions[msg.sender] += msg.value;
+    }
+
+    function receive() public payable {
+        contributions[msg.sender] += msg.value;
+    }
+}
+
+contract Fallback is FakeFallback {
+    receive() external payable {
+        contributions[msg.sender] += msg.value;
+    }
+
+    fallback() external payable {
+        contributions[msg.sender] += msg.value;
+    }
+}

tests/unit/core/test_fallback_receive.py

@@ -0,0 +1,20 @@
+from pathlib import Path
+from solc_select import solc_select
+
+from slither import Slither
+from slither.core.declarations.function import FunctionType
+
+TEST_DATA_DIR = Path(__file__).resolve().parent / "test_data"
+
+
+def test_fallback_receive():
+    solc_select.switch_global_version("0.6.12", always_install=True)
+    file = Path(TEST_DATA_DIR, "fallback.sol").as_posix()
+    slither = Slither(file)
+    fake_fallback = slither.get_contract_from_name("FakeFallback")[0]
+    real_fallback = slither.get_contract_from_name("Fallback")[0]
+
+    assert fake_fallback.fallback_function is None
+    assert fake_fallback.receive_function is None
+    assert real_fallback.fallback_function.function_type == FunctionType.FALLBACK
+    assert real_fallback.receive_function.function_type == FunctionType.RECEIVE