[ci] Silence warnings when running lib/tasks/test_webui.rake rake tasks

That's already the case for current OBS (running on rails 4.2 and rake v10.4),
and needs to be explicitly disabled with upgrading OBS to rails 5.

This caused our logs in travis to be spammed with warning messages,
like "warning: loading in progress, circular require considered harmful", after
switching to rails 5.
That circular require issue should be fixed nonetheless, eventually.

src/api/lib/tasks/test_webui.rake

@@ -6,6 +6,7 @@ Rake::TestTask.new do |t|
   test_files += FileList['test/**/*_test.rb'].exclude(%r{webui}).exclude(%r{test/models}).exclude(%r{test/unit})
   t.test_files = test_files
   t.name = 'test:api'
+  t.warning = false
 end
 
 # Everything with webui in the file name/path is the webui test suite,
@@ -21,6 +22,7 @@ Rake::TestTask.new do |t|
   filelist = filelist.exclude("test/functional/webui/spider_test.rb")
   t.test_files = filelist
   t.name = 'test:webui'
+  t.warning = false
 end
 
 # The spider test are in their own test suite to not pollute code coverage measurement.
@@ -29,4 +31,5 @@ Rake::TestTask.new do |t|
   proxy_mode_files = FileList.new
   t.test_files = proxy_mode_files.include("test/functional/webui/spider_test.rb")
   t.name = 'test:spider'
+  t.warning = false
 end