[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/8833429216191075777
Cq-Cl-Tag: roller-builder:pigweed-experimental-roller
Cq-Cl-Tag: roller-bid:8833429216191075777
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I93d9234d40483ae9e828f1cabb9ed49b05008e32
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/65523
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