[unittest] Add option to allow disabling sharding in unittest (#67063)

[unittest] Add lit option to allow disabling sharding in unittest

By default, googletest based unit tests uses sharding to speed up the
testing. However, when these unit tests are run through wrapper program
on a remote platform with large round trip time, the sharding will increase
the time cost dramatically. This patch adds a LLVM LIT option
"--disable-gtest-sharding" to allow sharding on gtest based unittest to
be disabled.