set bats_lib_path in test helper

The installation mechanism of dependencies is unique to a project,
and so the configuration of BATS_LIB_PATH should be owned by the test
suite, not assumed set as an outside env var.

test/test_helper.bash

diff --git a/test/test_helper.bash b/test/test_helper.bash
index beb9298..8d476a8 100644
--- a/test/test_helper.bash
+++ b/test/test_helper.bash
@@ -1,4 +1,5 @@
 # Load dependencies.
+BATS_LIB_PATH=$PWD/node_modules:${BATS_LIB_PATH-}
 bats_load_library 'bats-support'
 
 # Load library.