updated doc [skip ci]
fixed console guide and docs of FixtureController
Minor text adjustments
Docs about testing
Rewritten testing introduction, added environment setup to README.md, removed empty pages that were not referenced
expanded db-migration doc
Update db-migrations.md

fix a typo.
Fixes #4800.
fix bugs of BlameableBehavior in console application. In console application, \yii\console\Application doesn't have `getUser()` method. Use BlameableBehavior in console application will cause an exception.
Fixes #4736