Expand PHPUnit test coverage for Drupal\Core\Batch\BatchStorage

Problem/Motivation

BatchStorage lacks unit testing.

Proposed resolution

Add PHPUnit-based unit tests for BatchStorage.

Remaining tasks

• check other procedural calls and identify which are wrappers
• mock the database, #2002116: Convert core/modules/update/lib/Drupal/update/Tests/UpdateCoreUnitTest.php to phpunit → has an example of a mock, also see: http://phpunit.de/manual/3.7/en/test-doubles.html
• more TBD

User interface changes

No.

API changes

TBD (no?)

Related Issues

• #2003736: Convert system module's Batch unit tests to phpunit →
• #1938068: Convert UnitTestBase to PHPUnit →