Switched to Bats-core as a submodule

Can't use the official v0.4.0 release, since our tests rely on a change
that was made after the release of v0.4.0.

This code currently uses an unversioned commit of the bats-core repo, we
will switch to a supported version once it has been released, e.g.
v0.4.1.

test/run

@@ -1,12 +1,15 @@
 #!/usr/bin/env bash
 test_directory="$(cd "$(dirname "$0")" && pwd)"
-bats_executable="${test_directory}/../bats/bin/bats"
+#bats_executable="${test_directory}/../bats/bin/bats"
+bats_executable="${test_directory}/../test_lib/bats-core/bin/bats"
 
 [ ! -e $bats_executable ] && \
-  git clone --depth 1 https://github.com/sstephenson/bats.git ${test_directory}/../bats
+  git submodule init && git submodule update
 
 if [ -z "${BASH_IT}" ]; then
   export BASH_IT=$(cd ${test_directory} && dirname $(pwd))
 fi
 
+echo "Bats: $bats_executable"
+
 exec $bats_executable ${CI:+--tap} ${test_directory}/{bash_it,completion,install,lib,plugins,themes}