Copy trace fixture files, rather than symlinking

Since we are using tmpdirs in order to enable parallel testing, we
can go the full hog and drop the fakefs too and just copy all the
files directly into the tmpdir. If a test makes changes (e.g.
changing the icon file during an import) the copy in the tmpdir is
thrown away at the end of the test anyway.

test/controllers/api/traces_controller_test.rb

@@ -1,5 +1,4 @@
 require "test_helper"
-require "minitest/mock"
 
 module Api
   class TracesControllerTest < ActionController::TestCase

test/controllers/traces_controller_test.rb

@@ -1,5 +1,4 @@
 require "test_helper"
-require "minitest/mock"
 
 class TracesControllerTest < ActionController::TestCase
   # Use temporary directories with unique names for each test