Re-sort #include lines for unittests. This uses a slightly modified

clang-format (https://reviews.llvm.org/D33932) to keep primary headers
at the top and handle new utility headers like 'gmock' consistently with
other utility headers.

No other change was made. I did no manual edits, all of this is
clang-format.

This should allow other changes to have more clear and focused diffs,
and is especially motivated by moving some headers into more focused
libraries.

llvm-svn: 304786