nijie: fix parallel test failures.

Nijie tests fail often under parallel testing. This is because every
test needs to login to Nijie first, but Nijie rate-limits the login
endpoint, so eventually we hit the limit and tests start failing.

This is made worse by a thundering herd problem. Eight test processes
try to login to Nijie at the same time, but only one succeeds, so the
rest sleep and try again, but they all wakeup and try again at the same
time, hitting the rate limits again.

The workaround is to set the retry limit ridiculously high, higher than
we would ideally like in production. Another workaround would be to
serialize the Nijie tests in the test suite. This can be done with
lockfiles and flock(2). This helps, but we can still hit the rate limit
even under serialized execution.

app/logical/sources/strategies/nijie.rb

@@ -182,7 +182,7 @@ module Sources
         form = { email: Danbooru.config.nijie_login, password: Danbooru.config.nijie_password }
 
         # XXX `retriable` must come after `cache` so that retries don't return cached error responses.
-        response = http.cache(1.hour).use(:retriable).post("https://nijie.info/login_int.php", form: form)
+        response = http.cache(1.hour).use(retriable: { max_retries: 20 }).post("https://nijie.info/login_int.php", form: form)
         DanbooruLogger.info "Nijie login failed (#{url}, #{response.status})" if response.status != 200
         return nil unless response.status == 200