Simplify code in DrupalSqlBaseTest

Problem/Motivation

While working on #3151732: DrupalSqlBase::checkRequirements should test version with $minimum_version, we noticed some things in the test class Drupal\Tests\migrate_drupal\Unit\source\DrupalSqlBase that should be cleaned up.

Before that issue, there was already some duplicated code between the two test methods. Now there are three test methods that have this similar code.

Steps to reproduce

Proposed resolution

• Reduce duplicated code in the test methods. Either add one or more helper function or a setUp() method.
• Remove the second namespace declaration before TestDrupalSqlBase.
• Combine all the use statements at the start of the file.

Remaining tasks

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet

N/A