Add blank lines after module-level //! comments.

Most modules have such a blank line, but some don't. Inserting the blank
line makes it clearer that the `//!` comments are describing the entire
module, rather than the `use` declaration(s) that immediately follows.

tests/lang_tests_common.rs

@@ -1,4 +1,5 @@
 //! The common code for `tests/lang_tests_*.rs`
+
 use std::{
     env::{self, current_dir},
     path::{Path, PathBuf},