[roll third_party/pigweed] pw_unit_test: Update metadata test type for unit tests

In anticipation of generating metadata for additional test types, this
CL tweaks the existing test metadata, renaming the "type" field to
"test_type" and the "test" value to "unit_test".

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/154550

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 5ea607ffef8f3e6..a712de7e0273c99
Roller-URL: https://ci.chromium.org/b/8774943672864137585
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I281fb4c3c8063afd9c816255c2f7df5107f4683a
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/157891
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 5ea607f..a712de7 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 5ea607ffef8f3e6089a8dcbdbc57e4ccfa2bb930
+Subproject commit a712de7e0273c997e70d761ead21529ff575e502