[roll third_party/pigweed] pw_unit_test: Add serial test runner

Introduces a script that automates the process of flashing a test to a
device, triggering the test start, and evaluating the output to
determine if the tests passed or failed.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/154391

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 9b3b32c1a006d6f..c71c11f9619b5b6
Roller-URL: https://ci.chromium.org/b/8775742877903430273
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ib240316476728ef67f817699cdf2a573c5d11da8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/155092
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 9b3b32c..c71c11f 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 9b3b32c1a006d6fd7fe2476972b3351019946a79
+Subproject commit c71c11f9619b5b64852aeb5613ee17753c1813eb