Add return typehints to protected test helper methods

We could simplify this by using existing rector rules and create child issues that target one or more of these per issue. The relevant rules are listed in \Rector\Config\Level\TypeDeclarationLevel::RULES and I've identified the following as possibly relevant.

void:

• AddVoidReturnTypeWhereNoReturnRector

bool:

• ReturnTypeFromStrictBoolReturnExprRector
• BoolReturnTypeFromStrictScalarReturnsRector

array:

• ReturnTypeFromStrictNewArrayRector
• ReturnTypeFromReturnDirectArrayRector

The downfall is where rector determines the return type based on Other Things, one rule may end up with multiple return types.

• NumericReturnTypeFromStrictScalarReturnsRector
• ReturnTypeFromStrictConstantReturnRector
• ReturnTypeFromStrictTernaryRector
• ReturnTypeFromStrictScalarReturnExprRector
• ReturnTypeFromReturnNewRector
• ReturnTypeFromReturnCastRector
• ReturnTypeFromSymfonySerializerRector
• ReturnTypeFromStrictTypedPropertyRector
• AddReturnTypeDeclarationFromYieldsRector
• ReturnTypeFromStrictNativeCallRector
• ReturnTypeFromStrictTypedCallRector
• ReturnUnionTypeRector
• ReturnTypeFromStrictParamRector
• AddReturnTypeDeclarationBasedOnParentClassMethodRector
• ReturnTypeFromStrictFluentReturnRector
• ReturnNeverTypeRector

Added related issue ✨ Enforce return types in all new methods and functions RTBC for discussion.