soong_zip: Add tests

Add test that cover basic command line usage of soong_zip.  -D
is not covered yet as the implementation will be replaced with
one that is also more easily testable in the next patch.

Bug: 116751500
Test: zip_test.go
Change-Id: I5a1bcee74ebc9cb3cf332c36f89bc12c0e807ad2
diff --git a/cmd/merge_zips/merge_zips.go b/cmd/merge_zips/merge_zips.go
index 95ff70b..f383de9 100644
--- a/cmd/merge_zips/merge_zips.go
+++ b/cmd/merge_zips/merge_zips.go
@@ -250,7 +250,12 @@
 			addMapping(jar.MetaDir, dirSource)
 		}
 
-		fh, buf, err := jar.ManifestFileContents(manifest)
+		contents, err := ioutil.ReadFile(manifest)
+		if err != nil {
+			return err
+		}
+
+		fh, buf, err := jar.ManifestFileContents(contents)
 		if err != nil {
 			return err
 		}