Convert newtype_enum and struct_enum tests into modules

Separate testing each variant kind of enum (unit, newtype, tuple, struct) results
in more specific information if that checks fail

(review this commit with "ignore whitespace changes" option on)

test_suite/tests/test_enum_internally_tagged.rs

@@ -187,8 +187,11 @@ fn newtype_struct() {
     );
 }
 
-#[test]
-fn newtype_enum() {
+mod newtype_enum {
+    use super::*;
+
+    #[test]
+    fn unit() {
         assert_tokens(
             &InternallyTagged::NewtypeEnum(Enum::Unit),
             &[
@@ -200,7 +203,10 @@ fn newtype_enum() {
                 Token::MapEnd,
             ],
         );
+    }
 
+    #[test]
+    fn newtype() {
         assert_tokens(
             &InternallyTagged::NewtypeEnum(Enum::Newtype(1)),
             &[
@@ -212,7 +218,10 @@ fn newtype_enum() {
                 Token::MapEnd,
             ],
         );
+    }
 
+    #[test]
+    fn tuple() {
         // Reaches crate::private::de::content::VariantDeserializer::tuple_variant
         // Content::Seq case
         // via ContentDeserializer::deserialize_enum
@@ -233,7 +242,10 @@ fn newtype_enum() {
                 Token::MapEnd,
             ],
         );
+    }
 
+    #[test]
+    fn struct_() {
         // Reaches crate::private::de::content::VariantDeserializer::struct_variant
         // Content::Map case
         // via ContentDeserializer::deserialize_enum
@@ -271,6 +283,7 @@ fn newtype_enum() {
                 Token::MapEnd,
             ],
         );
+    }
 }
 
 #[test]
@@ -301,8 +314,11 @@ fn struct_() {
     );
 }
 
-#[test]
-fn struct_enum() {
+mod struct_enum {
+    use super::*;
+
+    #[test]
+    fn unit() {
         assert_de_tokens(
             &Enum::Unit,
             &[
@@ -356,6 +372,7 @@ fn struct_enum() {
                 Token::SeqEnd,
             ],
         );
+    }
 }
 
 #[test]