aconfig: add test utilities

Create a test utility function to create a Cache from the files in
testdata/*. A follow-up CL will update the unit tests to use this
instead of creating their own caches.

Bug: 283910447
Test: atest aconfig.test
Change-Id: Ice5064eadb0babde5eb38d292330d213ab136d96
diff --git a/tools/aconfig/src/main.rs b/tools/aconfig/src/main.rs
index c07bcf3..b3b6ac4 100644
--- a/tools/aconfig/src/main.rs
+++ b/tools/aconfig/src/main.rs
@@ -33,6 +33,9 @@
 mod commands;
 mod protos;
 
+#[cfg(test)]
+mod test;
+
 use crate::cache::Cache;
 use commands::{DumpFormat, Input, OutputFile, Source};