1158 Updating CEP18 Testing Docs

[ACStoneCL]

What does this PR fix/introduce?

Reintroducing testing framework documentation. The previous docs were no longer relevant to the new testing framework, now they are up to date. Closes #1158

Additional context

New CEP-18 Testing Framework Tutorial #1158

Checklist

(Delete any that aren't relevant)

• [x] For new internal links I used relative file paths (with .md extension) - e.g. ../../faq/faq-general.md - instead of introducing absolute file path, or relative/absolute URL.
• [x] All external links have been verified with yarn run check:externals.
• [x] My changes follow the Casper docs style guidelines.
• [x] All technical procedures have been tested (if you want help with this, mention it in Reviewers).

Reviewers

Editing: @ipopescu / Tech: @deuszex