runtime: make TestGcSys actually test something
runtime: make TestGcSys actually test something

The workthegc function was being inlined, and the slice did not
escape, so there was no memory allocation. Use a sink variable to
force memory allocation, at least for now.

Fixes #23343

Change-Id: I02f4618e343c8b6cb552cb4e9f272e112785f7cf
Reviewed-on: https://go-review.googlesource.com/122576
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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