Use file_test instead of custom sh_test (#255)

Address review comment from #254
diff --git a/tests/rules/BUILD b/tests/rules/BUILD
index b6d0df1..da6d406 100644
--- a/tests/rules/BUILD
+++ b/tests/rules/BUILD
@@ -1,8 +1,13 @@
+load(
+    "@bazel_tools//tools/build_rules:test_rules.bzl",
+    "file_test",
+)
+
 # Simply ensures the file supposed to be downloaded by the gce_file
-# test rule was actually downloaded
-sh_test(
+# test rule was actually downloaded and the contents of the file was
+# "0"
+file_test(
     name = "gcs_file_test",
-    srcs = [":file_existence_test.sh"],
-    args = ["$(location @download_test_gcs_file//file:test.txt)"],
-    data = ["@download_test_gcs_file//file:test.txt"],
+    content = "0",
+    file = "@download_test_gcs_file//file:test.txt",
 )
diff --git a/tests/rules/file_existence_test.sh b/tests/rules/file_existence_test.sh
deleted file mode 100755
index 5b113a7..0000000
--- a/tests/rules/file_existence_test.sh
+++ /dev/null
@@ -1,31 +0,0 @@
-#!/bin/bash
-#
-# Copyright 2017 The Bazel Authors. All rights reserved.
-#
-# Licensed under the Apache License, Version 2.0 (the "License");
-# you may not use this file except in compliance with the License.
-# You may obtain a copy of the License at
-#
-#    http://www.apache.org/licenses/LICENSE-2.0
-#
-# Unless required by applicable law or agreed to in writing, software
-# distributed under the License is distributed on an "AS IS" BASIS,
-# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-# See the License for the specific language governing permissions and
-# limitations under the License.
-
-# Script to run the executable generated from a docker_toolchain_autoconfig rule
-# and then check the toolchain configs for the c++ auto generated config exist.
-#
-# This script should be passed in 'srcs' of a sh_test test rule. The sh_test
-# rule is expected to have the name {docker_toolchain_autoconfig_name}_test,
-# where {docker_toolchain_autoconfig_name} is the docker_toolchain_autoconfig
-# rule you would like to build and run.
-
-set -e
-
-if [ -e $1 ]; then
-  echo "Verified $1 exists"
-else
-  echo "$1 did not exist" && false
-fi