Clean up imports, formatting and tests.

Bug: n/a
Test: atest libapkverify.test libapkverify.integration_test
Change-Id: I39686d17927741297aa3feb02ed6043ee770933b
diff --git a/idsig/src/hashtree.rs b/idsig/src/hashtree.rs
index a4727a9..b91698b 100644
--- a/idsig/src/hashtree.rs
+++ b/idsig/src/hashtree.rs
@@ -191,7 +191,7 @@
 
 #[cfg(test)]
 mod tests {
-    use crate::hashtree::*;
+    use super::*;
     use ring::digest;
     use std::fs::{self, File};