Deflake bigtable and spanner tests. [(#2224)](GoogleCloudPlatform/python-docs-samples#2224)

* Spanner doesn't actually promise the order of the results, so make the assertion work regardless of ordering.
* Bigtable might need some more time to scale, so retry the assertion up to 10 times.

Author: theacodes

samples/samples/snippets_test.py

@@ -202,7 +202,9 @@ def test_query_with_struct(capsys):
 def test_query_with_array_of_struct(capsys):
     snippets.query_with_array_of_struct(INSTANCE_ID, DATABASE_ID)
     out, _ = capsys.readouterr()
-    assert 'SingerId: 8\nSingerId: 7\nSingerId: 6' in out
+    assert 'SingerId: 8' in out
+    assert 'SingerId: 7' in out
+    assert 'SingerId: 6' in out
 
 
 def test_query_struct_field(capsys):