Add support for GTest based UnitTests

[delroth]

No support for MSVC at the moment. Fairly basic but should be enough to start adding tests for a few things.

[Sonicadvance1]

LGTM

[lioncash]

Looks good. I'm alright with this.

[comex]

[delroth]

@comex I'm planning to at least try and unittest a few modules that are not strongly coupled to the rest of Dolphin. For the rest, I'll be waiting for @degasus' null video backend to implement some larger scale tests (+ reusing the fifoci framework for some GFX tests).

[BhaaLseN]

I'm not terribly happy that you put up to 3 statements on one line (almost missed the EQ in the last test and wanted to complain about it)...but then again, yay, tests! \o/

[delroth]

.Yeah, I'm not a fan of the 3 statements per line either... I found it more readable than the alternative 3 lines though. I could in theory templatize that, but I'd prefer to keep tests simple.

I could fix it if you suggest a better way.

@neobrain expressed concerns about having tests built by default. At this point I think it is not a huge problem and we can revisit this decision later if tests start taking a long time to build. Ideally we would build tests only when running "make test", but I can't find a good way to do that (might have to ask on a CMake ML). Opinion on that?

[delroth]

Now tests are not built by default and are only built when trying to run the testsuite with "make unittests". Unfortunately, because cmake is a dumb tool, "make test" is and will be broken - there is no way around that at the moment (according to CMake devs).

[Parlane]

LGTM