The current SetGCPercent test is, shall we say, minimal.

Expand it to check that the GC target is actually computed and updated
correctly.

For #19076.

Change-Id: I6e9b2ee0ef369f22f72e43b58d89e9f1e1b73b1b
Reviewed-on: https://go-review.googlesource.com/39834
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>