[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
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https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: e59fb801bd8584c..5b96d29bb8a6ff0
Roller-URL: https://ci.chromium.org/b/8748983824864031345
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Change-Id: Ie1c843f5fa1c2b67d927629e8c839a3bbeeb384f
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/208114
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Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
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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