Add more tests for struct_field_align_threshold and trailing_comma

tests/source/issue-4791/trailing_comma.rs

@@ -0,0 +1,14 @@
+// rustfmt-struct_field_align_threshold: 30
+// rustfmt-trailing_comma: Always
+
+struct Foo {
+    group_a: u8,
+
+    group_b: u8
+}
+
+struct Bar {
+    group_a: u8,
+
+    group_b: u8,
+}

tests/target/issue-4791/no_trailing_comma.rs

@@ -0,0 +1,8 @@
+// rustfmt-struct_field_align_threshold: 0
+// rustfmt-trailing_comma: Never
+
+pub struct Baz {
+    group_a: u8,
+
+    group_b: u8
+}

tests/target/issue-4791/trailing_comma.rs

@@ -0,0 +1,14 @@
+// rustfmt-struct_field_align_threshold: 30
+// rustfmt-trailing_comma: Always
+
+struct Foo {
+    group_a: u8,
+
+    group_b: u8,
+}
+
+struct Bar {
+    group_a: u8,
+
+    group_b: u8,
+}