bundle exec rake assets:precompile shouldn't fail quietly.

If JavaScript runtime is not installed, execjs fails with error quietly,
while tests continue to run. This should not happen since it causes tests
to fail for unknown reason (#6621).

This commit assures that if JavaScript runtime is not installed, an assertion
is raised.

railties/test/application/assets_test.rb

@@ -17,9 +17,21 @@ def teardown
       teardown_app
     end
 
-    def precompile!
+    def precompile!(env = nil)
       quietly do
-        Dir.chdir(app_path){ `bundle exec rake assets:precompile` }
+        precompile_task = 'bundle exec rake assets:precompile'
+        precompile_task += ' ' + env if env
+        out = Dir.chdir(app_path){  %x[ #{precompile_task} ] }
+        assert $?.exitstatus == 0,
+               "#{precompile_task} has failed: #{out}.\
+                Probably you didn't install JavaScript runtime."
+        return out
+      end
+    end
+
+    def clean_assets!
+      quietly do
+        assert Dir.chdir(app_path){ system('bundle exec rake assets:clean') }
       end
     end
 
@@ -253,9 +265,8 @@ class ::PostsController < ActionController::Base ; end
       # digest is default in false, we must enable it for test environment
       add_to_env_config "test", "config.assets.digest = true"
 
-      quietly do
-        Dir.chdir(app_path){ `bundle exec rake assets:precompile RAILS_ENV=test` }
-      end
+      precompile!('RAILS_ENV=test')
+
       file = Dir["#{app_path}/public/assets/application.css"].first
       assert_match(/\/assets\/rails\.png/, File.read(file))
       file = Dir["#{app_path}/public/assets/application-*.css"].first
@@ -285,9 +296,9 @@ class ::PostsController < ActionController::Base ; end
       add_to_config "config.assets.compile = true"
 
       ENV["RAILS_ENV"] = nil
-      quietly do
-        Dir.chdir(app_path){ `bundle exec rake assets:precompile RAILS_GROUPS=assets` }
-      end
+
+      precompile!('RAILS_GROUPS=assets')
+
       file = Dir["#{app_path}/public/assets/application-*.css"].first
       assert_match(/\/assets\/rails-([0-z]+)\.png/, File.read(file))
     end
@@ -310,9 +321,7 @@ class ::PostsController < ActionController::Base ; end
       app_file "public/assets/application.css", "a { color: green; }"
       app_file "public/assets/subdir/broken.png", "not really an image file"
 
-      quietly do
-        Dir.chdir(app_path){ `bundle exec rake assets:clean` }
-      end
+      clean_assets!
 
       files = Dir["#{app_path}/public/assets/**/*", "#{app_path}/tmp/cache/assets/*"]
       assert_equal 0, files.length, "Expected no assets, but found #{files.join(', ')}"
@@ -440,9 +449,8 @@ class ::PostsController < ActionController::Base ; end
       add_to_config "config.assets.compile = true"
       add_to_config "config.assets.digest = true"
 
-      quietly do
-        Dir.chdir(app_path){ `bundle exec rake assets:clean assets:precompile` }
-      end
+      clean_assets!
+      precompile!
 
       files = Dir["#{app_path}/public/assets/application-*.js"]
       assert_equal 1, files.length, "Expected digested application.js asset to be generated, but none found"
@@ -453,9 +461,8 @@ class ::PostsController < ActionController::Base ; end
       add_to_env_config "production", "config.assets.prefix = 'production_assets'"
 
       ENV["RAILS_ENV"] = nil
-      quietly do
-        Dir.chdir(app_path){ `bundle exec rake assets:clean` }
-      end
+
+      clean_assets!
 
       files = Dir["#{app_path}/public/production_assets/application.js"]
       assert_equal 0, files.length, "Expected application.js asset to be removed, but still exists"