Distribution: Include tests in dist file

Since commit e948509 ("setup.py - don't distribute tests"), tests files
are not included in the `.tar.gz` bundle on a fresh repo clone. (On old
repos they were still included, because listed in
`yamllint.egg-info/SOURCES.txt`.)

Let's explicitly include them.

MANIFEST.in

@@ -1,3 +1,4 @@
 include LICENSE
 include README.rst
 include docs/*
+include tests/*.py tests/rules/*.py tests/yaml-1.2-spec-examples/*

setup.py

@@ -44,8 +44,7 @@ setup(
 
     packages=find_packages(exclude=['tests', 'tests.*']),
     entry_points={'console_scripts': ['yamllint=yamllint.cli:run']},
-    package_data={'yamllint': ['conf/*.yaml'],
+    package_data={'yamllint': ['conf/*.yaml']},
-                  'tests': ['yaml-1.2-spec-examples/*']},
     install_requires=['pathspec >=0.5.3', 'pyyaml'],
     test_suite='tests',
 )