test: add test for get_private_keys

inputs/bob.pub

@@ -0,0 +1,3 @@
+-----BEGIN CRYPT4GH PUBLIC KEY-----
+gPghilyXu+cZy1Iyego3yzZYi5vILe4ZulWDdL4CbGM=
+-----END CRYPT4GH PUBLIC KEY-----

inputs/bob.sec

@@ -0,0 +1,3 @@
+-----BEGIN CRYPT4GH PRIVATE KEY-----
+YzRnaC12MQAEbm9uZQAEbm9uZQAgroAjTA5BPeGfYZBwkk3dmiiIsYBTCvOgDsJVf2X9y3c=
+-----END CRYPT4GH PRIVATE KEY-----

tests/decryption/test_decrypt.py

@@ -0,0 +1,37 @@
+"""Tests for decrypt.py"""
+from pathlib import Path
+import pytest
+
+from crypt4gh.keys import get_private_key as get_pk_bytes
+from decrypt import get_private_keys
+
+INPUT_DIR = Path(__file__).parents[2]/"inputs"
+
+
+class TestGetPrivateKeys:
+    """Test get_private_keys."""
+
+    def test_empty_list(self):
+        """Test empty input."""
+        assert not get_private_keys([])
+
+    def test_invalid_path(self):
+        """Test input with an invalid filename."""
+        with pytest.raises(FileNotFoundError):
+            get_private_keys([INPUT_DIR/'bad_file_path'])
+
+    @pytest.mark.parametrize("files,expected_keys_retrieved", [
+        (["alice.pub", "bob.pub", "hello.c4gh"], []),
+        (["alice.sec", "alice.pub", "bob.pub"], ["alice.sec"]),
+        (["alice.sec", "bob.sec", "alice.pub", "bob.pub"], ["alice.sec", "bob.sec"])
+    ])
+    def test_get_keys(self, files, expected_keys_retrieved):
+        """Test ability to retrieve keys from a list of files.
+
+        Ensure that `get_private_keys` gets only private keys from a list of files.
+        """
+        def get_expected_bytes():
+            return [get_pk_bytes(INPUT_DIR/filename, callback=lambda x: '')
+                    for filename in expected_keys_retrieved]
+
+        assert get_private_keys([INPUT_DIR/file for file in files]) == get_expected_bytes()