Add tests for rebase_dump_accessibility_tree_test.py

The most substantial changes are in ParseLog(), which has changed to handle
parsing a single failing test rather than an entire logfile. (Splitting the log
file and writing the new file are moved to helper functions, which allows for
easier unit testing).

The script needs much more cleanup, but it is intentionally changed as little as
 possible in order to ensure solid test coverage before subsequent cleanup of
 the script.

The script is also renamed to rebase_dump_accessibility_tree_testS.py everywhere.


Change-Id: I9531e6aaf8ef183347439fa9a0184409eb2f1d1c
Bug: b:323348541
AX-relnotes: n/a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5666297
Commit-Queue: Jonny Wang <[email protected]>
Reviewed-by: Mark Schillaci <[email protected]>
Reviewed-by: Aaron Leventhal <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1321901}
diff --git a/docs/accessibility/browser/tests.md b/docs/accessibility/browser/tests.md
index d2c8358..a2373938 100644
--- a/docs/accessibility/browser/tests.md
+++ b/docs/accessibility/browser/tests.md
@@ -72,7 +72,7 @@
 Expectation baselines for each OS can be generated via:
 
 ```
-tools/accessibility/rebase_dump_accessibility_tree_test.py
+tools/accessibility/rebase_dump_accessibility_tree_tests.py
 ```
 
 ## Other content_browsertests