Fix print_elements test

The current implementation of gdb.Value.__str__ doesn't print children,
and the test wasn't checking that the prettyprinter yielded any
children, so it wasn't really testing anything.

Use the `p` command instead from the test as a workaround (so children
get printed), and add a positive CHECK to verify that the printer yields
as many elements as desired, but not more.

Author: slackito

test/lit/print_elements.test

@@ -18,4 +18,5 @@ still prints the "..." at the end of the children list if it's truncated.
 In this case, for 'set print elements 10' we would print children 0-9 and also
 yield child 10. So check that we don't yield 11.
 
+CHECK: yielding child 10
 CHECK-NOT: yielding child 11

test/lit/print_elements/__init__.py

@@ -24,7 +24,7 @@ def children(self):
     size = int(self.val["size"])
     i = 0
     while i < size:
-      print("yielding child %d" % i, file=sys.stderr)
+      print("yielding child %d" % i)
       yield "[%d]"%i, i
       i += 1
 
@@ -50,4 +50,4 @@ def __lldb_init_module(debugger, internal_dict):
   debugger.HandleCommand('settings set target.max-children-count 10')
   print("gdb.parameter('print elements'):", gdb.parameter('print elements'))
 
-  print("s = %s" % gdb.parse_and_eval('s'))
+  debugger.HandleCommand('p s')