[roll third_party/pigweed] pw_unit_test: Fix and test RPC service

- Update pw_unit_test's pw_rpc server streaming call to the new API.
- Fix issue where the test runner would crash if the first test was
  disabled.
- Add test that runs unit tests in a C++ binary from Python with the
  unit test RPC server.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/55511

third_party/pigweed Rolled-Commits: af7f5ef93da4372..81017fe1af1f6a9
Roller-URL: https://ci.chromium.org/b/8839787774908724368
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8839787774908724368
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I2e3c6b3c69848f16d422669e4fe1e1d25234a398
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/56127
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index af7f5ef..81017fe 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit af7f5ef93da43726f9dce144f1209a0c2b800c02
+Subproject commit 81017fe1af1f6a985c8634af8864ea64ecfeb9c3