Use make_mock_top function

Followup cleanup CL to address the comments from https://android-review.git.corp.google.com/c/platform/build/soong/+/2757787

Bug: b/300498226
Change-Id: If07e1fb21e97bd4b7550d29eaf7736b5894d797f
diff --git a/tests/lib.sh b/tests/lib.sh
index 50b0ef8..cb920c6 100644
--- a/tests/lib.sh
+++ b/tests/lib.sh
@@ -16,7 +16,7 @@
 if [[ -n "$HARDWIRED_MOCK_TOP" ]]; then
   MOCK_TOP="$HARDWIRED_MOCK_TOP"
 else
-  MOCK_TOP=$(mktemp -t -d st.XXXXX)
+  MOCK_TOP=$(make_mock_top)
   trap cleanup_mock_top EXIT
 fi