runtime/debug: expand SetGCPercent test
    
    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>