time: update TestSub to avoid future regressions
time: update TestSub to avoid future regressions

CL 131196 optimized Time.Sub, but was reverted because
it incorrectly computed the nanoseconds in some edge cases.
This CL adds a test case to enforce the correct behavior
so that a future optimization does not break this again.

Updates #17858
Updates #33677

Change-Id: I596d8302ca6bf721cf7ca11cc6f939639fcbdd43
Reviewed-on: https://go-review.googlesource.com/c/go/+/190524
Run-TryBot: Joe Tsai <7277d7402195d5c2006c89f45718cf82bb3e527f@gmail.com>
Reviewed-by: Daniel Martí <29bf28b42a938ace12a5819da4ed4ba03d82f315@mvdan.cc>
Reviewed-by: Andrew Bonventre <682ad9f487f614f19e8b7cc5d340bdc8e1c1b128@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,cloudflare/go,CAFxX/go,nawawi/go,golang/go,golang/go,akutz/go,golang/go,akutz/go,CAFxX/go,golang/go,CAFxX/go,akutz/go,CAFxX/go,cloudflare/go,nawawi/go,nawawi/go,cloudflare/go,akutz/go,akutz/go,nawawi/go,golang/go,cloudflare/go,nawawi/go,akutz/go,cloudflare/go,golang/go,CAFxX/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,nawawi/go,nawawi/go,akutz/go,golang/go,nawawi/go,akutz/go,cloudflare/go,cloudflare/go