build: Enable Checkstyle checks for test code

[valfirst]

Change list

• Move Checkstyle config to a dedicated folder
• Configure Checkstyle suppressions for test code
• Fix all Checkstyle violations in test code
• Enable check for Checkstyle violation in test code during build

Types of changes

What types of changes are you proposing/introducing to Java client?
Put an x in the boxes that apply

• No changes in production code.
• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)