use session.evaluate instead of session.parse(...).evaluate (#1543)

Just a small fix to the `test/builtin/drawing/test_plot_detail.py`:
using `session.evaluate` ensures that trailing messages from previous
tests are cleaned before the new evaluation.
This was the reason for the last failure in CI for #1538

Author: mmatera

test/builtin/drawing/test_plot_detail.py

@@ -139,8 +139,7 @@ def one_test(name, str_expr, vec, opt, act_dir="/tmp"):
 
     try:
         # evaluate the expression to be tested
-        expr = session.parse(str_expr)
-        act_expr = expr.evaluate(session.evaluation)
+        act_expr = session.evaluate(str_expr)
         if len(session.evaluation.out):
             print("=== messages:")
             for message in session.evaluation.out: