Adjusted tests so that stubbed `ContainerInterface#has()` returns `bool` instead of `null`

[Ocramius]

This is necessary, as psr/container:^2 has a stricter return type declaration, and prophecy is not respecting the return types upfront (unless stubbed methods are explicitly configured).

While this is an upstream BC break in psr/container:^2, this component is not really affected by the BC issue, other than some stubbing being broken in it.

Fixes:

1) LaminasTest\InputFilter\InputFilterPluginManagerFactoryTest::testFactoryConfiguresPluginManagerUnderContainerInterop with data set "input" ('Laminas\InputFilter\InputInterface')
TypeError: Double\ContainerInterface\P8::has(): Return value must be of type bool, null returned
2) LaminasTest\InputFilter\InputFilterPluginManagerFactoryTest::testFactoryConfiguresPluginManagerUnderContainerInterop with data set "input-filter" ('Laminas\InputFilter\InputFilt...erface')
TypeError: Double\ContainerInterface\P8::has(): Return value must be of type bool, null returned

Ref: https://github.com/laminas/laminas-servicemanager/issues/146#issue-1310587609 Ref: https://github.com/laminas/laminas-servicemanager/issues/146#issuecomment-1189956888

Q	A
Documentation	no
Bugfix	yes
BC Break	no
New Feature	no
RFC	no
QA	yes

[Ocramius]

Note: this is a bugfix that only affects tests, so I'm YOLO-merging it and releasing once CI is green.

[Ocramius]

Also: going to old-stable branch so that bugfix patches aren't automatically red