Steps to reproduce the issue

1. Create a test env for php, php-memcached and hhvm and apply PR #11565.

2. sudo service memcached restart

3. View libraries/vendor/phpunit/phpunit/phpunit. It should start like:
   #!/usr/bin/env php

4. Run libraries/vendor/phpunit/phpunit/phpunit tests/unit/suites/libraries/joomla/cache/storage/JCacheStorageMemcachedTest

   PHPUnit 4.8.24 by Sebastian Bergmann and contributors.
   Warning: The Xdebug extension is not loaded
   No code coverage will be generated.

   .......

   Time: 5.08 seconds, Memory: 8.00MB

   OK (7 tests, 17 assertions)

5. Edit libraries/vendor/phpunit/phpunit/phpunit and replace first like from php to hhvm.
   Should be: #!/usr/bin/env hhvm

6. Run libraries/vendor/phpunit/phpunit/phpunit tests/unit/suites/libraries/joomla/cache/storage/JCacheStorageMemcacheTest

   PHPUnit 4.8.24 by Sebastian Bergmann and contributors.
   Warning: The Xdebug extension is not loaded
   No code coverage will be generated.

   Fatal error: [] operator not supported for strings in libraries/joomla/cache/storage/memcache.php on line 194

7. Run sudo service memcached restart

8. (The same as point 6) Run libraries/vendor/phpunit/phpunit/phpunit tests/unit/suites/libraries/joomla/cache/storage/JCacheStorageMemcacheTest

   PHPUnit 4.8.24 by Sebastian Bergmann and contributors.
   Warning: The Xdebug extension is not loaded
   No code coverage will be generated.

   .......

   Time: 9.17 seconds, Memory: 28.64MB

   OK (7 tests, 17 assertions)

Expected result

Memcache Test for HHVM should pass on PR #11565

Actual result

but did not pass because it was run after memcached storage did test which left empty array in memcached server.

I suggest to restart memcached server before each memcache/memcached tests.

Summary

Memcache storage test fails if memcached server has value from memcached tests.

https://travis-ci.org/joomla/joomla-cms/jobs/152511380