Fix docs file name and add main() failure mode

Bug: 1145216
Change-Id: I9479c0e088ac4acf115f9da14e499268c9576504
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2618559Reviewed-by: Nodir Turakulov <nodir@chromium.org>
Commit-Queue: Nodir Turakulov <nodir@chromium.org>
Auto-Submit: Gregory Guterman <guterman@google.com>
Cr-Commit-Position: refs/heads/master@{#841792}

docs/testing/regression-test-selection.md → docs/testing/regression_test_selection.md

@@ -25,11 +25,16 @@ Test skipping happens at the GN level in
 [source_set](/build/config/BUILDCONFIG.gn) and [test](/testing/test.gni)
 GN targets.
 
-## Known failure mode
+## Known failure modes
 
-Consider a test file A that contains unit tests, as well as some variables
+There are not known to be many instances of these failure modes in the codebase.
+Those that are known are never excluded by our model.
+
+- **Shared state in test files**: Consider a test file A that contains unit tests, as well as some variables
 used in another file B. When our RTS strategy excludes A, but not B, a
 compilation error will occur.
+- **main() defined in test files**: A test file contains tests and the `main()` function for the entire suite.
+When it is excluded, the whole suite fails to compile.
 
 ## Design Docs