WebUI: Move generate_grd test files into separate subdirectory.

This cleans up ui/webui/resources/tools/tests, which holds the data
of multiple tests.

Change-Id: Ia991a3909b9960002ccc93d3cb8faf9be70895d5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4548369


Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Commit-Queue: Cole Horvitz <colehorvitz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1147381}