Rootcanal: do not starve the reader in unittest

On an M1 the writer thread can write faster than the reader can read.
This results in the reader never exiting the loop, causing the test to
hang.

We now lower the rate of writing to make sure the readers can always
keep up.

Bug: 251187705
Test: Unit tests now succeed on M1
Change-Id: Ia675c3b3b0613b2b00a588a803473a3d997bf0ee