[roll third_party/pigweed] pw_log: Add tests for Rust printf and println backends

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/194952

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: ee60eb2ee49d19b..64bfe3952506478
Roller-URL: https://ci.chromium.org/b/8754241367489559633
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I6782524b0737948635acb4445d6483c77615ce1f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/194965
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 ee60eb2..64bfe39 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit ee60eb2ee49d19b8a867c94b6cb8837cedadd6aa
+Subproject commit 64bfe39525064789c2f79efa9f2d9afe06a0af0c