Remove testing/ and third_party/googletest/, build gn_unittests

- Add support for building gn_unittests.
- Add a minimal googletest-alike in src/test/test.h (and remove
  third_party/googletest).
- Switch to explicitly listing all source files in build/gen.py because
  the globs were a mess (and wrong).
- Add .clang-format file.

Change-Id: I1a4ea2114d176750ba9050a2dbf48b89aad6575c
Reviewed-on: https://gn-review.googlesource.com/1340
Reviewed-by: Brett Wilson <brettw@chromium.org>