Remove `tests` module from installation. (#564)

diff --git a/setup.py b/setup.py
index 5a4937c..3b065c8 100755
--- a/setup.py
+++ b/setup.py
@@ -59,7 +59,7 @@
         license='Apache2.0',
         url='https://github.com/google/mobly',
         download_url='https://github.com/google/mobly/tarball/1.8.1',
-        packages=setuptools.find_packages(),
+        packages=setuptools.find_packages(exclude=['tests']),
         include_package_data=False,
         scripts=['tools/sl4a_shell.py', 'tools/snippet_shell.py'],
         tests_require=[