build: skip building tests when BUILD_TESTING=OFF

[chenrui333]

coverage on master

Summary of changes

Summarize what you changed

Rationale for changes

Why did you make these changes?

Additional info and certifications

This pull request (PR) is a:

• Bug fix
• Feature addition
• Other, Please describe: skip unit tests when BUILD_TESTING=OFF

I certify that

• I certify that:
  • I have reviewed and followed the contributing guidelines
  • I will wait at least 24 hours before self-approving the PR to give another
    OpenCoarrays developer a chance to review my proposed code
  • I have not introduced errant white space (no trailing white space or white space errors may
    be introduced)
  • I have added an explanation of what these changes do and why they should be included
  • I have checked to ensure there aren't other open Pull Requests for the same change
  • I have you written new tests for these changes
  • I have successfully tested these changes locally
  • I have commented any non-trivial, non-obvious code changes
  • The commits are logically atomic, self consistent and coherent
  • The commit messages follow best practices
  • Test coverage is maintained or increased after this is merged

Code coverage data

---

relates to Homebrew/homebrew-core#230634

[rouson]

@chenrui333 my apologies for the long delay in responding to this PR. There currently is no one funded to maintain OpenCoarrays. Please let me know if you would like to have the required privileges for merging this PR. If so, could you recommend someone to review it? Currently, there's one other open PR #800 (where I just put a similar comment) submitted by another potential contributor. How about the two of you review and approve each other's PRs before merging?