[llvm][lit] Respect GTEST_TOTAL_SHARDS and GTEST_SHARD_INDEX env vars

There are a variety of issues with using GTest sharding by default for users of
`lit` using the Google Test formatter as mentioned in
https://github.com/llvm/llvm-project/issues/56492 and
https://github.com/llvm/llvm-project/issues/56491.

Currently, there is no way for users to explicitly control the sharding
behavior, even with the environment variables that GTest provides. This patch
teaches the `googletest` formatter to actually respect `GTEST_TOTAL_SHARDS`
and `GTEST_SHARD_INDEX` environment variables if they are set.

In practice, we could go one step further and not do any of the post-processing
of the JSON files if `GTEST_TOTAL_SHARDS` is `1` for example, but that it left
as a follow-up if desired.  There may be preferred alternative approaches to
disabling sharding entirely through another mechanism, such as a lit config
variable.

Differential Revision: https://reviews.llvm.org/D133542