better assertions for bitfield behavior

tests/pragmas/tbitsize.nim

@@ -1,7 +1,18 @@
 type
   bits* = object
-    flag* {.bitsize: 1.}: cint
+    flag* {.bitsize: 1.}: cuint
     opts* {.bitsize: 4.}: cint
 
-var b: bits
-echo b.flag
+var
+  b: bits
+
+assert b.flag == 0
+b.flag = 1
+assert b.flag == 1
+b.flag = 2
+assert b.flag == 0
+
+b.opts = 7
+assert b.opts == 7
+b.opts = 9
+assert b.opts == -7