[clang-format][NFC] Format unit tests with insert/remove braces

This patch is the result of running clang-format version 753fe330 in
clang/unittests/Format/:
clang-format -style="{InsertBraces: true, RemoveBracesLLVM: true}" -i *.cpp *.h

Differential Revision: https://reviews.llvm.org/D125510