tests: add unit-tests for calculate_etag()

Add test coverage for `calucalte_etag` that cover the two cases
(with or without multipart).

tests/unit/module_utils/test_s3.py

@@ -5,10 +5,52 @@
 # GNU General Public License v3.0+ (see COPYING or https://www.gnu.org/licenses/gpl-3.0.txt)
 
 from __future__ import (absolute_import, division, print_function)
+
 __metaclass__ = type
 
 from ansible_collections.amazon.aws.tests.unit.compat.mock import MagicMock
 from ansible_collections.amazon.aws.plugins.module_utils import s3
+from ansible.module_utils.basic import AnsibleModule
+
+import pytest
+
+
+class FakeAnsibleModule(AnsibleModule):
+    def __init__(self):
+        pass
+
+
+def test_calculate_etag_single_part(tmp_path_factory):
+    module = FakeAnsibleModule()
+    my_image = tmp_path_factory.mktemp("data") / "my.txt"
+    my_image.write_text("Hello World!")
+
+    etag = s3.calculate_etag(
+        module, str(my_image), etag="", s3=None, bucket=None, obj=None
+    )
+    assert etag == '"ed076287532e86365e841e92bfc50d8c"'
+
+
+def test_calculate_etag_multi_part(tmp_path_factory):
+    module = FakeAnsibleModule()
+    my_image = tmp_path_factory.mktemp("data") / "my.txt"
+    my_image.write_text("Hello World!" * 1000)
+
+    mocked_s3 = MagicMock()
+    mocked_s3.head_object.side_effect = [{"ContentLength": "1000"} for _ in range(12)]
+
+    etag = s3.calculate_etag(
+        module,
+        str(my_image),
+        etag='"f20e84ac3d0c33cea77b3f29e3323a09-12"',
+        s3=mocked_s3,
+        bucket="my-bucket",
+        obj="my-obj",
+    )
+    assert etag == '"f20e84ac3d0c33cea77b3f29e3323a09-12"'
+    mocked_s3.head_object.assert_called_with(
+        Bucket="my-bucket", Key="my-obj", PartNumber=12
+    )
 
 
 def test_validate_bucket_name():