golang_str: tests: Robustify xbytes/xunicode/xbytearray

Assert that input belongs to the set of expected types.
Assert that the output has exactly the type we promised.

No change in functionality. We are now just more certain that those
functions work as intended and could be relied upon.

golang/golang_str_test.py

@@ -2719,9 +2719,29 @@ def bench_bencode(b):
 
 # xbytes/xunicode/xbytearray convert provided bytes/unicode object to bytes,
 # unicode or bytearray correspondingly to function name.
-def xbytes(x):     return x.encode('utf-8') if type(x) is unicode else x
-def xunicode(x):   return x.decode('utf-8') if type(x) is bytes   else x
-def xbytearray(x): return bytearray(xbytes(x))
+def xbytes(x):
+    assert isinstance(x, (bytes,unicode))
+    if isinstance(x, unicode):
+        x = x.encode('utf-8')
+    assert isinstance(x, bytes)
+    x = _bdata(x)
+    assert type(x) is bytes
+    return x
+
+def xunicode(x):
+    assert isinstance(x, (bytes,unicode))
+    if isinstance(x, bytes):
+        x = x.decode('utf-8')
+    assert isinstance(x, unicode)
+    x = _udata(x)
+    assert type(x) is unicode
+    return x
+
+def xbytearray(x):
+    assert isinstance(x, (bytes,unicode))
+    x = bytearray(xbytes(x))
+    assert type(x) is bytearray
+    return x
 
 # deepReplaceStr2Bytearray replaces str to bytearray, or hashable-version of
 # bytearray, if str objects are detected to be present inside set or dict keys.