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/test_gen.rs

@@ -892,52 +892,3 @@ pub struct RemotePackedNonCopyDef {
 impl Drop for RemotePackedNonCopyDef {
     fn drop(&mut self) {}
 }
-
-//////////////////////////////////////////////////////////////////////////
-
-/// Regression tests for <https://github.com/serde-rs/serde/issues/2371>
-pub mod static_and_flatten {
-    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,
-        },
-    }
-}