Add shebang lines to tests directory shell scripts (#288)

diff --git a/tests/copy_file/copy_file_tests.sh b/tests/copy_file/copy_file_tests.sh
index dfee635..90d79ee 100755
--- a/tests/copy_file/copy_file_tests.sh
+++ b/tests/copy_file/copy_file_tests.sh
@@ -1,3 +1,5 @@
+#!/bin/bash
+
 # Copyright 2019 The Bazel Authors. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
diff --git a/tests/diff_test/diff_test_tests.sh b/tests/diff_test/diff_test_tests.sh
index ed894c2..5af60aa 100755
--- a/tests/diff_test/diff_test_tests.sh
+++ b/tests/diff_test/diff_test_tests.sh
@@ -1,3 +1,5 @@
+#!/bin/bash
+
 # Copyright 2019 The Bazel Authors. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
diff --git a/tests/write_file/write_file_tests.sh b/tests/write_file/write_file_tests.sh
index 2464230..247c696 100755
--- a/tests/write_file/write_file_tests.sh
+++ b/tests/write_file/write_file_tests.sh
@@ -1,3 +1,5 @@
+#!/bin/bash
+
 # Copyright 2019 The Bazel Authors. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");