Fix help, add RPC tests for getblockheader

- Add assert_is_hex_string and assert_is_hash_string to RPC test utils.
- Add RPC documentation for getblockheader[chainwork].
- Add RPC tests for getblockheader.

Github-Pull: #7194
Rebased-From: 16d4fce0b2 4745636126 135d6ec8ce

qa/rpc-tests/test_framework/util.py

@@ -407,6 +407,23 @@ def assert_raises(exc, fun, *args, **kwds):
     else:
         raise AssertionError("No exception raised")
 
+def assert_is_hex_string(string):
+    try:
+        int(string, 16)
+    except Exception as e:
+        raise AssertionError(
+            "Couldn't interpret %r as hexadecimal; raised: %s" % (string, e))
+
+def assert_is_hash_string(string, length=64):
+    if not isinstance(string, basestring):
+        raise AssertionError("Expected a string, got type %r" % type(string))
+    elif length and len(string) != length:
+        raise AssertionError(
+            "String of length %d expected; got %d" % (length, len(string)))
+    elif not re.match('[abcdef0-9]+$', string):
+        raise AssertionError(
+            "String %r contains invalid characters for a hash." % string)
+
 def satoshi_round(amount):
     return  Decimal(amount).quantize(Decimal('0.00000001'), rounding=ROUND_DOWN)