Fix siteinfo/service test if build dir path contained multiple occurences of 'build/'

Replace only the _last_ occurrence of 'build/' in the path to
service_test.js to get the path to the source tree.

When run under Travis CI for example, the full path to the source dir
is '/home/travis/build/robertknight/1pass-web/build/lib/siteinfo/service_test.js'