Android: some minor improvements to jni_generator_tests.py

Addresses some comments to jni_generator_tests.py.

BUG=

Review URL: https://codereview.chromium.org/101613002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238785 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238785