Gather comments in the preamble of a test file.

Comments preceding the first function are not associated with any
specific entity in the file. Put them in a TestFile::preamble_comments
field.

lib/reader/src/lib.rs

@@ -10,7 +10,7 @@ extern crate cretonne;
 pub use error::{Location, Result, Error};
 pub use parser::{parse_functions, parse_test};
 pub use testcommand::{TestCommand, TestOption};
-pub use testfile::{TestFile, Details};
+pub use testfile::{TestFile, Details, Comment};
 pub use isaspec::IsaSpec;
 pub use sourcemap::SourceMap;