Fix duplicate absl::Hash seed in component builds via test_support.

In component builds, `//third_party/abseil-cpp:absl` already includes
all compiled dependencies of `absl/hash:hash_testing` (e.g `absl/hash`
), while `hash_testing` and `spy_hash_state` themselves are header-only.

However, `//third_party/abseil-cpp:test_support` unconditionally added
`//third_party/abseil-cpp/absl/hash:hash_testing` to `public_deps`.
Because `hash_testing` and its transitive dependency `absl/hash` are
`source_set` targets, test executables depending on `:test_support`
(such as `components_unittests`) statically linked a second copy of
`hash.o` and `absl::hash_internal::MixingHashState::kSeed` directly
into the executable in addition to the copy exported by
`libthird_party_abseil-cpp_absl.dylib`.

Because `MixingHashState::Seed()` uses the address of `kSeed` (`&kSeed`)
as its per-process hash seed, hash tables such as `google::protobuf::Map`
and `absl::flat_hash_map` populated inside a shared library (e.g.
`ParseFromString` in `libprotobuf`) hashed keys using the dylib's
`kSeed`, while lookups inlined into the test executable (`.contains()`,
`.find()`) hashed keys using the executable's duplicate `kSeed`, causing
cross-DSO map lookups to fail in `components_unittests`.

Update `test_support` in component builds to expose the `hash_testing`
headers on top of `:absl` and googletest instead of pulling in the
`absl/hash:hash_testing` `source_set` graph a second time.

Detailed Context, root cause and investigation: http://gpaste/6755399435830337

Change-Id: I3c3e146ddef12758eeb122a4a18a85f53348afba
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/8619338
Reviewed-by: Danil Chapovalov <danilchap@chromium.org>
Commit-Queue: Jingyun Liu <jingyun@google.com>
Cr-Commit-Position: refs/heads/main@{#1724216}
NOKEYCHECK=True
GitOrigin-RevId: c0edc85d9062c05aef03ead4f4d121f1e60f096c