SDO-923: Add check for symlink function existence

[Chrico]

Please check if the PR fulfills these requirements

• The commit message follows our guidelines
• Tests for the changes have been added (for bug fixes/features)
• Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Check if symlink function exists before proceeding.

Other information

• Some hosters do not support symlink creation via PHP which leads to:

PHP message: PHP Fatal error: Uncaught Error: Call to undefined function symlink() in ....

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.15%. Comparing base (795ffaf) to head (27cce65).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master      #71   +/-   ##
=========================================
  Coverage     88.15%   88.15%           
  Complexity      343      343           
=========================================
  Files            27       27           
  Lines           979      979           
=========================================
  Hits            863      863           
  Misses          116      116           

Flag	Coverage Δ
unittests	88.15% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[o-samaras]

Thanks