Add pretty-printer test of tuple field function call

Author: dtolnay

tests/ui-fulldeps/pprust-parenthesis-insertion.rs

@@ -77,6 +77,9 @@ static EXPRS: &[&str] = &[
     // These mean different things.
     "if let _ = true && false {}",
     "if let _ = (true && false) {}",
+    // Parentheses to call a named field, but not an unnamed field.
+    "(self.fun)()",
+    "(self.0)()",  // FIXME: no parenthesis needed.
     // Conditions end at the first curly brace, so struct expressions need to be
     // parenthesized. Except in a match guard, where conditions end at arrow.
     "if let _ = (Struct {}) {}",