completion test for string union type

[shruti2522]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• [ ] N
• [x] Y

re #1391

2. What is the scope of this PR (e.g. component or file name):

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• [ ] Affects user behaviors
• [ ] Contains syntax changes
• [ ] Contains variable changes
• [ ] Contains experimental features
• [ ] Performance regression: Consumes more CPU
• [ ] Performance regression: Consumes more Memory
• [ ] Other

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• [ ] N
• [ ] Y

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• [x] Unit test
• [ ] Integration test
• [ ] Benchmark (add benchmark stats below)
• [ ] Manual test (add detailed scripts or steps below)
• [ ] Other

[coveralls]

Pull Request Test Coverage Report for Build 9477369131

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 71.105%

---

Totals
Change from base Build 9477229425:	0.03%
Covered Lines:	55057
Relevant Lines:	77431

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 9477381337

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 71.105%

---

Totals
Change from base Build 9477229425:	0.03%
Covered Lines:	55057
Relevant Lines:	77431

---

💛 - Coveralls