test(fmt): Verify frontmatter is not stripped

Author: epage

Diff for: src/tools/rustfmt/tests/source/frontmatter.rs

@@ -0,0 +1,11 @@
+
+
+---   identifier
+[dependencies]
+regex = "1"
+
+---
+
+#![feature(frontmatter)]
+
+fn main() {}

Diff for: src/tools/rustfmt/tests/target/frontmatter.rs

@@ -0,0 +1,9 @@
+---   identifier
+[dependencies]
+regex = "1"
+
+---
+
+#![feature(frontmatter)]
+
+fn main() {}