Add GTest support

[sethfowler]

We really need to start writing more unit tests. One reason that keeps getting put off is that we've agreed that we want to use GTest, but nobody has had the time to actually integrate GTest into the build system. I took yesterday's hackathon as an opportunity to do so.

e925048 adds GTest as a submodule.

4bba2ac adds GTest support to the build system. Tests can be added from any Makefile by adding them to gtest_SOURCES. All GTests will be linked into a single GTest executable, gtestp4c. gtestp4c will be automatically run during make check, or you can run it directly if you want to use GTest's nice features.

To kick things off, I translated an existing unit test (opeq_test) into a GTest.

[sethfowler]

I should note that I've confirmed that a failing GTest triggers a make check failure and that the log shows up in test-suite.log. I've also tested adding additional GTests from extensions.

[mihaibudiu]

We have maybe 8 unit tests, but we have a few hundred compiler tests.
Will the latter take advantage of gtest in any way?

[sethfowler]

@mbudiu-vmw the answer to that question right now is "maybe", which isn't very satisfying, I know. =)

We've talked about whether we could gain some benefits from using GTest to drive the compiler tests. Making that change could let us get rid of some hacks, but it's probably not worth putting a lot of work into, so I think the current consensus is "we'll do it if it's very simple". We don't know yet whether it's simple, though, because nobody has actually tried to do it.

It's true that we only have 8 unit tests, by the way, but I'm hoping to encourage people to write more. =)

[sethfowler]

The test failed due to cpplint. Doh. I'll push a fix.

[ChrisDodd]

So what's the upside of this? It doesn't look like gtest provides any extra functionality we don't already have, just requires writing different boilerplate in the test, and doesn't really save or improve anything.

[antoninbas]

@ChrisDodd many pre-defined test assertions, value-parametrized tests, type-parametrized tests, exception support,... I would say all of these things are pretty useful if you guys plan on writing unit tests more systematically.

[sethfowler]

Thanks! I'll rebase this and get it landed later today.