Allow //external/bazel-skylib to use existing BUILD files

Bug: 190605804
Test: bp2build-incremental
Change-Id: Ida765465532c0e98d035eef4c703d13fbf554538
diff --git a/tests/lib.sh b/tests/lib.sh
index 1d9b8d4..813a9dd 100644
--- a/tests/lib.sh
+++ b/tests/lib.sh
@@ -113,6 +113,7 @@
 
   symlink_directory prebuilts/bazel
   symlink_directory prebuilts/jdk
+  symlink_directory external/bazel-skylib
 
   symlink_file WORKSPACE
   symlink_file BUILD