update custom scheme tests to utilize pytest fixture

This isolates the implementation in response to PR feedback:
#467 (comment)

tests/test_client.py

@@ -4,9 +4,11 @@
 import string
 from pathlib import Path
 
+import pytest
+
 from cloudpathlib import CloudPath
 from cloudpathlib.client import register_client_class
-from cloudpathlib.cloudpath import register_path_class
+from cloudpathlib.cloudpath import implementation_registry, register_path_class
 from cloudpathlib.s3.s3client import S3Client
 from cloudpathlib.s3.s3path import S3Path
 
@@ -121,32 +123,47 @@ def my_content_type(path):
         _test_write_content_type(suffix, content_type, rig)
 
 
-@register_path_class("mys3")
-class MyS3Path(S3Path):
-    cloud_prefix: str = "mys3://"
+@pytest.fixture
+def custom_s3_path():
+    # A fixture isolates these classes as they modify the global registry of
+    # implementations.
+    @register_path_class("mys3")
+    class MyS3Path(S3Path):
+        cloud_prefix: str = "mys3://"
+
+    @register_client_class("mys3")
+    class MyS3Client(S3Client):
+        pass
+
+    yield (MyS3Path, MyS3Client)
 
+    # cleanup after use
+    implementation_registry.pop("mys3")
 
-@register_client_class("mys3")
-class MyS3Client(S3Client):
-    pass
 
+def test_custom_mys3path_instantiation(custom_s3_path):
+    CustomPath, _ = custom_s3_path
 
-def test_custom_mys3path_instantiation():
-    path = MyS3Path("mys3://bucket/dir/file.txt")
-    assert isinstance(path, MyS3Path)
+    path = CustomPath("mys3://bucket/dir/file.txt")
+    assert isinstance(path, CustomPath)
     assert path.cloud_prefix == "mys3://"
     assert path.bucket == "bucket"
     assert path.key == "dir/file.txt"
 
 
-def test_custom_mys3client_instantiation():
-    client = MyS3Client()
-    assert isinstance(client, MyS3Client)
+def test_custom_mys3client_instantiation(custom_s3_path):
+    _, CustomClient = custom_s3_path
+
+    client = CustomClient()
+    assert isinstance(client, CustomClient)
     assert client.CloudPath("mys3://bucket/dir/file.txt").cloud_prefix == "mys3://"
 
 
-def test_custom_mys3client_default_client():
-    MyS3Client().set_as_default_client()
+def test_custom_mys3client_default_client(custom_s3_path):
+    _, CustomClient = custom_s3_path
+
+    CustomClient().set_as_default_client()
+
     path = CloudPath("mys3://bucket/dir/file.txt")
-    assert isinstance(path.client, MyS3Client)
+    assert isinstance(path.client, CustomClient)
     assert path.cloud_prefix == "mys3://"