Move the test to postsubmit group

The test is failing on ToT as it still requires some extra build change.

Bug: 123886976
Test: None
Change-Id: Ibb1c54aaf68ed2dc06db4a7b1fc416f975c73fb0
diff --git a/TEST_MAPPING b/TEST_MAPPING
index 8a3c205..a3ede88 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -1,5 +1,5 @@
 {
-  "presubmit": [
+  "postsubmit": [
     {
       "name": "libavb_host_unittest",
       "host": true