Rename all tests in tests folder

Using the name 'main' was confusing

tests/end_to_end.rs

@@ -7,7 +7,7 @@ use std::io::Cursor;
 // This test asserts that after creating a zip file, then reading its contents back out,
 // the extracted data will *always* be exactly the same as the original data.
 #[test]
-fn main() {
+fn end_to_end() {
     let file = &mut Cursor::new(Vec::new());
 
     write_to_zip_file(file).expect("file written");

tests/invalid_date.rs

@@ -26,6 +26,6 @@ const BUF : &[u8] = &[
 ];
 
 #[test]
-fn main() {
+fn invalid_date() {
     let _archive = ZipArchive::new(Cursor::new(BUF)).unwrap();
 }