Rename WTF::{Arena|Tree}TestHelpers namepsace to WTF::{arena|tree}_test_helpers

https://google.github.io/styleguide/cppguide.html#Namespace_Names
> Namespace names are all lower-case.

This CL has no behavior changes.

Bug: 889726
Change-Id: I9dc96c9669ca28719ecda9d571eec0311d014a82
Reviewed-on: https://chromium-review.googlesource.com/c/1329711
Reviewed-by: Kent Tamura <tkent@chromium.org>
Commit-Queue: Zhuoyu Qian <zhuoyu.qian@samsung.com>
Cr-Commit-Position: refs/heads/master@{#607150}