[roll third_party/pigweed] pw_base64: Fix variable style; reduce tests

- Rename constexpr variables to use constant style.
- Reduce the number of randomly generated tests. There is plenty of
  with the reduced number of tests.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/65500

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 69285347b4bafd9..b4c6bb5533f6b75
Roller-URL: https://ci.chromium.org/b/8833429216726507345
Cq-Cl-Tag: roller-builder:pigweed-sample-project-roller
Cq-Cl-Tag: roller-bid:8833429216726507345
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Iafaa99d0b50cf73d5c7776de6d9db2e1ab28c0be
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/sample_project/+/65522
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 6928534..b4c6bb5 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 69285347b4bafd9f6e3c19c27085d6bac0491334
+Subproject commit b4c6bb5533f6b7586a9abcf578301e2ab91b6af7