[roll third_party/pigweed] pw_unit_test: Add stub Zephyr tests

Introduce a test directory compatible with Zephyr's Twister. When
invoked, the various tests from the testcase.yaml (currently one 1)
will be built and run on all available Zephyr platforms. In the first
pass, only the base64 module is added, but more can be added without
additional changes to the test (only adding the appropriate new entry
in testcase.yaml and the matching Kconfigs.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/148274

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 1e4339e6d21d1d1..7f101691a2f46d5
Roller-URL: https://ci.chromium.org/b/8780326462129013793
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I7b6fe9abe1b9a09305681b04ff57e81edd923759
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/148450
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 1e4339e..7f10169 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 1e4339e6d21d1d1b68402e5e51ff9cd9e381ac4e
+Subproject commit 7f101691a2f46d5f555db4201a3e1d79757646a0