All tests iOS unit tests build on the bots and pass locally when using gn.

We should run the tests on the bots to avoid regression. We won't be able to drop support for gyp as iOS still uses gyp for downstream builds.

The main difference between gyp and gn is that iossim is located in $root_build_dir/clang_x64/iossim instead of $root_build_dir/iossim.

dpranke: can you take care of this?