chore(s3-deployment): restore lambda tests (aws#12220)

The change in aws#12129 accidentally deleted the unit tests for the s3-deployment lambda function. This change restores them.

Since tests are written in python, run them inside a docker image derived from public.ecr.aws/lambda/python.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-s3-deployment/test/lambda.test.ts

@@ -0,0 +1,7 @@
+import { spawnSync } from 'child_process';
+import * as path from 'path';
+
+test('lambda python pytest', () => {
+  const result = spawnSync(path.join(__dirname, 'lambda', 'test.sh'), { stdio: 'inherit' });
+  expect(result.status).toBe(0);
+});

packages/@aws-cdk/aws-s3-deployment/test/lambda/Dockerfile

@@ -0,0 +1,13 @@
+FROM public.ecr.aws/lambda/python:latest
+
+# add everything to /opt/awscli (this is where `aws` is executed from)
+ADD . /opt/awscli
+
+# install boto3, which is available on Lambda
+RUN pip3 install boto3
+
+# run tests
+WORKDIR /opt/awscli
+RUN ["python3", "./test.py"]
+
+ENTRYPOINT [ "/bin/bash" ]

packages/@aws-cdk/aws-s3-deployment/test/lambda/aws

@@ -0,0 +1,27 @@
+#!/usr/bin/env python
+#===================================================================================================
+# mock for the "aws" cli
+#
+# the mock behaves as follows:
+# - argvs are going to be written to "aws.out" (one command in each line)
+# - if "aws s3 cp" is invoked, the destination will be populated with a test zip file.
+# - for "cp" and "sync", "aws.out" argv[4] is replaced by "archive.zip" and "contents.zip"
+#   becuase the actual value is a full path of a temporary directory
+#
+import sys
+import json
+import os
+import shutil
+
+scriptdir=os.path.dirname(os.path.realpath(__file__))
+
+# if "cp" is called with a local destination, copy a test zip file to the destination or 
+if sys.argv[2] == "cp" and not sys.argv[4].startswith("s3://"):
+    shutil.copyfile(os.path.join(scriptdir, 'test.zip'), sys.argv[4])
+    sys.argv[4] = "archive.zip"
+
+if sys.argv[2] == "sync":
+    sys.argv[4 if '--delete' in sys.argv else 3] = "contents.zip"
+
+with open("./aws.out", "a") as myfile:
+    myfile.write(json.dumps(sys.argv[1:]) + "\n")