[TestNG] Print suggested snippets per scenario

[mpkorstanje]

Summary

Implements #1740 for TestNG. By including the snippet suggestion
each time a test fails because a step is undefined test runners that
group output per test case can clearly document which steps need
to be implemented for each scenario.

This will allow users to use Cucumber JUnit without including the
summary plugin. Allowing a bit more configuration to be replaced by
defaults.

Screenshots:

Types of changes

• Bug fix (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).

Checklist:

• I've added tests for my code.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.

[coveralls]

Coverage increased (+0.2%) to 87.751% when pulling bcb8a86 on include-steps-when-test-is-skipped into 20e8c8c on master.