Unit tests for Blockchain.verify_header.

The function Blockchain.verify_header was previously not covered by tests
at all.  Even removing all the tests in it would still make the unit tests
pass.  This change adds tests for this important (!) function.

electrum/tests/test_blockchain.py

@@ -339,3 +339,37 @@ class TestBlockchain(SequentialTestCase):
 
         for b in (chain_u, chain_l, chain_z):
             self.assertTrue(all([b.can_connect(b.read_header(i), False) for i in range(b.height())]))
+
+
+class TestVerifyHeader(SequentialTestCase):
+
+    # Data for Bitcoin block header #100.
+    valid_header = "0100000095194b8567fe2e8bbda931afd01a7acd399b9325cb54683e64129bcd00000000660802c98f18fd34fd16d61c63cf447568370124ac5f3be626c2e1c3c9f0052d19a76949ffff001d33f3c25d"
+    target = Blockchain.bits_to_target(0x1d00ffff)
+    prev_hash = "00000000cd9b12643e6854cb25939b39cd7a1ad0af31a9bd8b2efe67854b1995"
+
+    def setUp(self):
+        super().setUp()
+        self.header = deserialize_header(bfh(self.valid_header), 100)
+
+    def test_valid_header(self):
+        Blockchain.verify_header(self.header, self.prev_hash, self.target)
+
+    def test_expected_hash_mismatch(self):
+        with self.assertRaises(Exception):
+            Blockchain.verify_header(self.header, self.prev_hash, self.target,
+                                     expected_header_hash="foo")
+
+    def test_prev_hash_mismatch(self):
+        with self.assertRaises(Exception):
+            Blockchain.verify_header(self.header, "foo", self.target)
+
+    def test_target_mismatch(self):
+        with self.assertRaises(Exception):
+            other_target = Blockchain.bits_to_target(0x1d00eeee)
+            Blockchain.verify_header(self.header, self.prev_hash, other_target)
+
+    def test_insufficient_pow(self):
+        with self.assertRaises(Exception):
+            self.header["nonce"] = 42
+            Blockchain.verify_header(self.header, self.prev_hash, self.target)