testgrid: only load 4 days of test-go results
testgrid: only load 4 days of test-go results

We can't currently display 14 days of results, because of AppEngine timeouts and RAM limits. This should work better.
GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid,GoogleCloudPlatform/testgrid