llvm_tools: Unify patch json representation

At present, I made a mistake where patch_utils expected
certain keys to exist. However, the original patch_manager.py
didn't. This was further broken by the fact that patch_sync.py
would avoid serializing the 'platforms' field if empty, causing
a back and forth between patch_utils.py and patch_sync.

Update the unittests to match and verify we're doing the correct
thing.

BUG=None
TEST=./patch_utils_unittest.py
TEST=./patch_manager_unittest.py

Change-Id: Ib02c9d552848831f395b006de9a28ea4292b82f7
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/toolchain-utils/+/3704542
Tested-by: Jordan Abrahams-Whitehead <ajordanr@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
Commit-Queue: Jordan Abrahams-Whitehead <ajordanr@google.com>
3 files changed