DEV: Speed up /admin/docker/repos.json spec (#237)

Requesting /admin/docker/repos.json with a lot of plugins installed can
take a lot of time because of the large number of calls to the 'git'
CLI that are used to retrieve various information about the plugin
(eg. branch, latest commit, remote URL, etc).

This is not a problem in production because of memoization and the
much lower number of installed plugins, but it is a problem in test
environment where specs requesting this page can take 10+ seconds with
~100 installed plugins.

This commit speeds up by stubbing the `find_all` method to return only
the first few repositories.

spec/requests/admin_controller_spec.rb

@@ -38,6 +38,12 @@ RSpec.describe DockerManager::AdminController do
   end
 
   describe "#repos" do
+    before do
+      # Return only the first 3 repos to reduce the number of calls to 'git' CLI
+      repos = DockerManager::GitRepo.find_all
+      DockerManager::GitRepo.stubs(:find_all).returns(repos[0...3])
+    end
+
     it "should return the right response" do
       sign_in(Fabricate(:admin))