Clean the test path after each spec run

FactoryBot's build_stubbed ignores the current database sequence of the
projects table and starts at ID 1000. If more than 1000 projects are created
during a test run, leftover repositories can cause spec failures. For example,
a spec that expects an empty repository may fail since there may be existing
content.

Closes #5461

spec/spec_helper.rb

@@ -97,6 +97,10 @@ RSpec.configure do |config|
     TestEnv.init
   end
 
+  config.after(:all) do
+    TestEnv.clean_test_path
+  end
+
   config.before(:example) do
     # Skip pre-receive hook check so we can use the web editor and merge.
     allow_any_instance_of(Gitlab::Git::Hook).to receive(:trigger).and_return([true, nil])