build: Make unit tests execution a part of Gradle build lifecycle

[valfirst]

Change list

This change simplifies unit tests task management and execution:

• test task of type Test is created automatically
• in comparison with previous approach no need to invoke extra task (it was unitTest): whenever build task is run, test task will be executed (this happens because test task is attached to the check lifecycle task)

More details: https://docs.gradle.org/current/userguide/java_testing.html#sec:java_testing_basics

Also now:

• JitPack build is failed if unit tests fail: no reason to publish invalid build
• Successful run of unit tests is a mandatory prerequisite for execution of integration Android and iOS tests

Types of changes

What types of changes are you proposing/introducing to Java client?

• 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)