Add regression test

Author: Ruben Schmidmeister

tests/config/normalize_multiline_doc_attribute.toml

@@ -0,0 +1 @@
+normalize_doc_attributes = true

tests/source/normalize_multiline_doc_attribute.rs

@@ -0,0 +1,4 @@
+#[doc = "This comment
+is split
+on multiple lines"]
+fn foo() {}

tests/target/normalize_multiline_doc_attribute.rs

@@ -0,0 +1,4 @@
+///This comment
+///is split
+///on multiple lines
+fn foo() {}