Fix test/FunctionTest.cpp build breaks

Fix test/FunctionTest.cpp build breaks

Summary:
The Folly `make check` build is broken with Clang on OS X.

This diff fixes two breaks in `test/FunctionTest.cpp`:

1. `FunctionTest.cpp:39:20: error: implicit instantiation of undefined template 'std::__1::array<int, 100>'`
2. `FunctionTest.cpp:103:3: error: no matching conversion for functional-style cast from 'Function<int (int)>' to '::testing::AssertionResult'`

The first was a missing `#include <array>`, and the second is a workaround for this gtest bug:
https://github.com/google/googletest/issues/429

Reviewed By: djwatson

Differential Revision: D3361188

fbshipit-source-id: 8140de978a2cbaf0f4aab45781ce0d656f03202b