Applying labels to test cases does not appear to save when on the creation page

Summary

When creating a test case, you can add labels. The dropdown doesn't indicate that labels have been selected once you close it after selecting labels to add.

Steps to reproduce

1. Create test case
2. Add labels
3. Close label selector

Example Project

What is the current bug behavior?

The labels I add do not appear until I click the Create button.

What is the expected correct behavior?

I should be able to see the labels I've added, similarly to when we add labels to issues.

Relevant logs and/or screenshots

test-case-bug

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

List out the selected labels in the dropdown toggle as well as list them underneath, as we do with issues.