Make a directory dedicated to regression tests

I have had a good experience with this pattern in many of my other
libraries.

test_suite/tests/regression/issue2371.rs

@@ -0,0 +1,43 @@
+use serde::Deserialize;
+
+#[derive(Deserialize)]
+pub struct Nested;
+
+#[derive(Deserialize)]
+pub enum ExternallyTagged {
+    Flatten {
+        #[serde(flatten)]
+        nested: Nested,
+        string: &'static str,
+    },
+}
+
+#[derive(Deserialize)]
+#[serde(tag = "tag")]
+pub enum InternallyTagged {
+    Flatten {
+        #[serde(flatten)]
+        nested: Nested,
+        string: &'static str,
+    },
+}
+
+#[derive(Deserialize)]
+#[serde(tag = "tag", content = "content")]
+pub enum AdjacentlyTagged {
+    Flatten {
+        #[serde(flatten)]
+        nested: Nested,
+        string: &'static str,
+    },
+}
+
+#[derive(Deserialize)]
+#[serde(untagged)]
+pub enum UntaggedWorkaround {
+    Flatten {
+        #[serde(flatten)]
+        nested: Nested,
+        string: &'static str,
+    },
+}