fix char* coercion test

tests/run/ctypedef_char_types.pyx

@@ -22,13 +22,13 @@ def const_charptrs():
     my_st = st
     assert typeof(my_st) == "const_char *", typeof(my_st)
     obj = my_st
-    assert obj == b'XYZ'
+    assert obj == b'XYZ', obj
 
     assert typeof(ust) == "const_uchar *", typeof(ust)
     my_ust = ust
     assert typeof(my_ust) == "const_uchar *", typeof(my_ust)
     obj = my_ust
-    assert obj == b'XYZ'
+    assert obj == b'XYZ', obj
 
 ctypedef char mychar
 ctypedef unsigned char myuchar
@@ -41,17 +41,18 @@ def const_char_arrays():
     cdef object obj
     cdef mychar[4]  st
     cdef myuchar[4] ust
+    cdef char ch
 
     i = 0
-    for ch in <char*>b'XYZ\0':
+    for ch in b'XYZ\0':
         st[i] = ch
-        ust[i] = ch
+        ust[i] = <unsigned char>ch
         i += 1
 
     assert typeof(st) == "mychar [4]", typeof(st)
     obj = st
-    assert obj == b'XYZ'
+    assert obj == b'XYZ', obj
 
     assert typeof(ust) == "myuchar [4]", typeof(ust)
     obj = ust
-    assert obj == b'XYZ'
+    assert obj == b'XYZ', obj