Add support for downloading test data

The commit introduces a make target 'testdata' that downloads the
required test data from the WebM project website. The data will also
be downloaded if invoking `make test` but is not a strict requirement
for only building the test executable.

The download directory is taken from the LIBVPX_TEST_DATA_PATH
environment variable, or may be specified as part of the make command.
If unset, it defaults to the current directory. It's expected that
most developers will want to set this environment variable to a place
outside their source/build trees, to avoid having to download the data
more than once.

To add test data file:

  1) add a line to test/test.mk:

      LIBVPX_TEST_DATA-yes += foo-bar-file.y4m

  2) add its sha1sum to the test/test-data.sha1 file in the following
     format:

      528cc88c821e5f5b133c2b40f9c8e3f22eaacc4c  foo-bar-file.y4m

  3) upload the file to the website

      $ gsutil cp foo-bar-file.y4m gs://downloads.webmproject.org/test_data/libvpx

T...