Make symlink during test

The tests/test_tree/blueprint symlink causes find to print a warning
in the Android build:
find: File system loop detected; `./build/blueprint/tests/test_tree/blueprint' is part of the same file system loop as `./build/blueprint'.

Make the symlink when running the test instead.

Change-Id: I52788af7b262f8aa7a00a501f0b50695e469adb1