Delete net::TestDataStream(), and update the one test fixture using it.

TestDataStream creates a stream of integers with digits in reverse
order ("0.1.2.3.4.5.6.7.8.9.01.02.03...") that consumers can
(almost) endlessly pull from, and then had another streaming
method to verify that a stream matches that data. The latter was never
used.

Only spdy_session_unittest was using this class, and it was only
using it to create fixed-size data, never taking advantage of its
streaming capabilities. It also was putting the data in IOBuffers
that it never used as IOBuffers, and was never verifying the data read
from those buffers.

This CL migrates those tests to use single-character strings of
the desired lengths.

This CL also conveniently removes the `allow_unsafe_buffers` in
test_data_stream.cc, since it deletes that file.

Bug: 40284755
Change-Id: I2427ccbdee0f16a8c41fbf8b452fa4c40d453bd3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6505522
Reviewed-by: Adam Rice <ricea@chromium.org>
Commit-Queue: mmenke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1455385}