Use willReturn() in mocks instead of will(returnValue())

[morozov]

Q	A
Type	improvement
BC Break	no

The calls like $mock->will($this->returnValue($value)) are unnecessary verbose and may cause conflicts with 4.0.x which enforces static calls as self::returnValue().

Using the shortcut consistently will make it simpler to identify the no longer needed mocks in 4.0.x where PHPUnit will be able to generate them automatically based on the method return type.