test: Verify frontmatter unpretty behavior

Author: epage

tests/ui/unpretty/frontmatter.rs

@@ -0,0 +1,10 @@
+---
+---
+
+//@ compile-flags: -Zunpretty=normal
+//@ check-pass
+
+#![feature(frontmatter)]
+
+fn main() {
+}

tests/ui/unpretty/frontmatter.stdout

@@ -0,0 +1,7 @@
+
+//@ compile-flags: -Zunpretty=normal
+//@ check-pass
+
+#![feature(frontmatter)]
+
+fn main() {}