[roll third_party/pigweed] pw_digital_io_linux: Refactor test

This separates the mocked file descriptor (VFS) code from the digital IO
test, and provides improved flexibility and clarity of implementing new
files.

This also adds comments to the tests.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/196051
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: e59fb801bd8584c..5b96d29bb8a6ff0
Roller-URL: https://ci.chromium.org/b/8748947749972135121
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I9e54caac71405eaeaf32bbe64d08121236ac6454
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/kudzu/+/208215
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Lint: Lint 🤖 <android-build-ayeaye@system.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index e59fb80..5b96d29 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit e59fb801bd8584caea9258547b1161b34c0c183e
+Subproject commit 5b96d29bb8a6ff0e47393276da856f7ffbd6465d