Revisited the way Guice creates an injector during unit test

The motivation of this is to enable test code to inject components from Jenkins, which means we need a single Injector that can see all of those, including @TestExtension and regular @Extension. Then I realized that the current implementation is broken, as it creates two independent Injectors. This has other undesirable side effects like @TestExtension components not being able to inject @Extensions, etc.

I briefly experimented with hierarchical Injectors (where we first create Injector that loads @Extension then create another that loads @TestExtension as a child), but this was hard to get right as we have no control over which ExtensionFinder gets instantiated first --- we'd be at the mercy of SezPoz.

So the eventual approach is to allow a single GuiceFinder to recognize multiple annotations. This also works well with our using child injector for installing plugins without restart