[CodeQuality] Detect Behat context by interface, not class name suffix (#691)

isBehatContext() matched any class whose name ends with 'Context' via
str_ends_with(). That false-positives on production classes like
SchemaValidationContext, rewriting their assert() calls into
\PHPUnit\Framework\Assert:: calls and pulling PHPUnit into non-test code.

Detect a real Behat context by whether it implements
Behat\Behat\Context\Context instead.

Author: TomasVotruba