Provide a way to build and run unittests on Windows

[shuffle2]

UnitTests have been integrated into msbuild as a standalone project which is compiled by default, but not run. The tests can be run as you would normally run an output binary from VS, or you (or the buildbot) can pass /p:RunUnitTests=true to msbuild.
Note that the tests are currently failing on windows :)
In order to add more tests, you only add files under UnitTests directory, no modification to the msbuild files should be required.

[shuffle2]

@dolphin-emu-bot rebuild

[shuffle2]

@dolphin-emu-bot rebuild
windows should report build failure as result of unittests failing...

[delroth]

@dolphin-emu-bot rebuild (let's try again)

[delroth]

@shuffle2 ask to wait until we make the unittest compilation happen only if we want to run tests.

[shuffle2]

I also experimented with GoogleTest Runner which allows the builtin VS experience for gtest binaries. This will be the best way for developers to run tests locally, apart from using msbuild directly.

I'll need to think a bit about a nice way to separate the build and blah blah...

[delroth]

Can we move the build separation to a following PR?

Would also really appreciate some documentation about running tests on Windows, the GTest extension, etc. :)

[shuffle2]

@delroth It should be ready to merge now.
I decided not to have the "test" step only execute the tests...it will still compile if any changes are detected (nix/osx behaves the same way).

[shuffle2]

BTW for VS users reading this, the easiest way to get the gtest adapter is:
extensions and updates->online->visual studio gallery
search for "gtest" and install GoogleTest Runner.
Then reopen VS, and Test Explorer will be functional.