Fix while_loop test.

- add output tensor to main block
- fix bug where tensor cannot serialize list of const data

Signed-off-by: Kevin Cheng <kevin.cheng@arm.com>
Change-Id: I40b96d634642e24e6fae6fa3ad6846f5911005f7
diff --git a/verif/tosa_test_gen.py b/verif/tosa_test_gen.py
index 760ed06..942f75c 100644
--- a/verif/tosa_test_gen.py
+++ b/verif/tosa_test_gen.py
@@ -1798,6 +1798,7 @@
             [iter_out.name, a_out.name, acc_out.name],
             attr,
         )
+        self.ser.addOutputTensor(acc_out)
 
         # COND block (input: iter, output: cond_tens )
         self.ser.startBasicBlock(cond_block)