chromeos: Add file manager JS test for recentsMyFiles

As part of the go/crosier effort we're removing Tast tests from the
chromium commit queue. Per go/crosier-conversion we're either porting
these tests to C++ GTest tests in the chromium tree or we're extending
existing tests.

Tast test filemanager.RecentFilesAppear tests the "Recents" UI in the
files app. There are existing JS integration tests in recents.js that
cover most of what the Tast test covers. This CL adds coverage for the
"My Files" folder, so that recents.js now covers the same areas as the
Tast test.

I verified using a "step-by-step" run of the test that the files appear
in "my files" in the test.

Bug: b:314357474
Test: browser_tests --gtest_filter=*FilesAppBrowserTest*recentsMyFiles
Change-Id: I40eb31ceba3ba9f0d01bdc1d27cf9639a2b77a96
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5078484
Reviewed-by: Luciano Pacheco <lucmult@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1233037}