Fix Flaky test_execute_on_shift_enter (#1097)

* Wait for text to render in DOM before 'assert', to fix timing issue.

pyscriptjs/tests/integration/support.py

@@ -1,4 +1,5 @@
 import dataclasses
+import math
 import os
 import pdb
 import re
@@ -372,6 +373,31 @@ class PyScriptTest:
 
 # ============== Helpers and utility functions ==============
 
+MAX_TEST_TIME = 30  # Number of seconds allowed for checking a testing condition
+TEST_TIME_INCREMENT = 0.25  # 1/4 second, the length of each iteration
+TEST_ITERATIONS = math.ceil(
+    MAX_TEST_TIME / TEST_TIME_INCREMENT
+)  # 120 iters of 1/4 second
+
+
+def wait_for_render(page, selector, pattern):
+    """
+    Assert that rendering inserts data into the page as expected: search the
+    DOM from within the timing loop for a string that is not present in the
+    initial markup but should appear by way of rendering
+    """
+    re_sub_content = re.compile(pattern)
+    py_rendered = False  # Flag to be set to True when condition met
+
+    for _ in range(TEST_ITERATIONS):
+        content = page.inner_html(selector)
+        if re_sub_content.search(content):
+            py_rendered = True
+            break
+        time.sleep(TEST_TIME_INCREMENT)
+
+    assert py_rendered  # nosec
+
 
 class JsErrors(Exception):
     """