Set rust_test modules as NATIVE_TESTS

* Rust test modules should have NATIVE_TESTS class,
  not EXECUTABLES, to work with atest --host.

Bug: 140938178
Test: make rust projects; run atest --host .
Change-Id: Ie9e237ee8c17b6e2d41f08a77c2b79d462a2c193
diff --git a/rust/androidmk.go b/rust/androidmk.go
index 2a9a6c0..edd5c5f 100644
--- a/rust/androidmk.go
+++ b/rust/androidmk.go
@@ -89,6 +89,7 @@
 
 func (test *testDecorator) AndroidMk(ctx AndroidMkContext, ret *android.AndroidMkData) {
 	test.binaryDecorator.AndroidMk(ctx, ret)
+	ret.Class = "NATIVE_TESTS"
 	stem := String(test.baseCompiler.Properties.Stem)
 	if stem != "" && !strings.HasSuffix(ctx.Name(), "_"+stem) {
 		// Avoid repeated suffix in the module name.