[CodeQuality] Detect Behat context by interface, not class name suffix

[TomasVotruba]

Bug

AssertFuncCallToPHPUnitAssertRector decided a class was a Behat context with:

return str_ends_with($className, 'Context');

Any class whose name ends with Context matched — including ordinary
production classes that have nothing to do with Behat. The rule then rewrote
their assert() calls into \PHPUnit\Framework\Assert:: calls, pulling PHPUnit
into non-test source.

Found in webonyx/graphql-php, whose
rector.php skips this rule on src for exactly this reason. SchemaValidationContext
(a plain production class) was rewritten:

-assert($type instanceof ScalarType, 'only remaining option');
+\PHPUnit\Framework\Assert::assertInstanceOf(ScalarType::class, $type, 'only remaining option');

Fix

Detect a real Behat context by whether it implements Behat\Behat\Context\Context
(the existing BehatClassName::CONTEXT constant, already used the same way in
ScalarArgumentToExpectedParamTypeRector), instead of matching the class name.

Existing *Context fixtures updated to implement the interface; added
skip_non_behat_context_class.php.inc covering a non-Behat *Context class.