hash/maphash: add more tests for seed generation

hash/maphash: add more tests for seed generation

Test all the paths by which a Hash picks its seed.
Make sure they all behave identically to a preset seed.

Change-Id: I2f7950857697f2f07226b96655574c36931b2aae
Reviewed-on: https://go-review.googlesource.com/c/go/+/220686
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Vladimir Evgrafov <evgrafov.vladimir@gmail.com>
Reviewed-by: Alan Donovan <adonovan@google.com>