Add gtest as a git submodule in third_party and integrate it into the build the same way WABT does. Adds a new executable, `binaryen-unittests`, to execute `gtest_main`. As a nontrivial example test, port one of the `TypeBuilder` tests from example/ to gtest/. Using gtest has a number of advantages over the current example tests: - Tests are compiled and linked at build time rather than runtime, surfacing errors earlier and speeding up test execution. - Tests are all built into a single binary, reducing overall link time and further reducing test overhead. - Tests are built from the same CMake project as the rest of Binaryen, so compiler settings (e.g. sanitizers) are applied uniformly rather than having to be separately set via the COMPILER_FLAGS environment variable. - Using the industry-standard gtest rather than our own script reduces our maintenance burden. Using gtest will lower the barrier to writing C++ tests and will hopefully lead to us having more proper unit tests.