[roll third_party/pigweed] pw_unit_test: Create facade's header

- Introduce a framework header for the pw_unit_test facade instead of
  using gtest/gtest.h as a facade. This header provides the macros
  defined by gtest/gtest.h that are also supported in pigweed.
- Replace pw_unit_test_GOOGLETEST_BACKEND for pw_unit_test_BACKEND,
  which points to either pw_unit_test:light or pw_unit_test:googletest.
- Provide a pw_unit_test:googletest framework backend that implements
  the framework facade using GoogleTest. This will be needed when
  including gtest/gtest.h or any GoogleTest header directly.

Test: compile with and without googletest backend.
Original-Bug: 309665550

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/183194

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 70b05788999ebb5..17e3417a74749bc
Roller-URL: https://ci.chromium.org/b/8762435921668789249
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I17b6c1fa439dbb006789308e4cb544f0649fb5c8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/184215
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 70b0578..17e3417 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 70b05788999ebb5298edc6551ca99aa95e4524db
+Subproject commit 17e3417a74749bc2c554d7214297a0866d729de9