Move generated Travis test dirs outside the tree

This should prevent the running of Travis tests from causing
a filesystem loop src.test/test_tree/blueprint -> ../..

Bug: 63100441
Test: NINJA=... ./tests/test_tree_tests.sh; stat src.test 2>/dev/null \
      && echo "src.test should not exist"

Change-Id: Ie01fe8999c738dca166e3fabde9d9391f6b70210