runtime: more diagnostics for TestStackGrowth
runtime: more diagnostics for TestStackGrowth

This adds diagnostics so we can tell if the finalizer has started, in
addition to whether or not it has finished.

Updates #19381.

Change-Id: Icb7b1b0380c9ad1128b17074828945511a6cca5d
Reviewed-on: https://go-review.googlesource.com/45138
Run-TryBot: Austin Clements <7ea35d812706d9213868749011af1ed4fa2f6aa0@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

Cofyc/go,Cofyc/go,akutz/go,cloudflare/go,Cofyc/go,Cofyc/go,nawawi/go,vsekhar/elastic-go,polyverse-security/go,codestation/go,akutz/go,cloudflare/go,polyverse-security/go,codestation/go,vsekhar/elastic-go,akutz/go,akutz/go,Cofyc/go,cloudflare/go,codestation/go,josharian/go,nawawi/go,Cofyc/go,codestation/go,codestation/go,polyverse-security/go,nawawi/go,cloudflare/go,cloudflare/go,nawawi/go,golang/go,nawawi/go,cloudflare/go,christopher-henderson/Go,CAFxX/go,vsekhar/elastic-go,vsekhar/elastic-go,golang/go,CAFxX/go,vsekhar/elastic-go,josharian/go,codestation/go,josharian/go,cloudflare/go,polyverse-security/go,vsekhar/elastic-go,CAFxX/go,golang/go,polyverse-security/go,akutz/go,golang/go,josharian/go,Cofyc/go,akutz/go,golang/go,nawawi/go,josharian/go,nawawi/go,CAFxX/go,vsekhar/elastic-go,CAFxX/go,cloudflare/go,akutz/go,golang/go,golang/go,CAFxX/go,Cofyc/go,golang/go,josharian/go,polyverse-security/go,codestation/go,CAFxX/go,polyverse-security/go,nawawi/go,akutz/go,CAFxX/go,vsekhar/elastic-go,josharian/go,polyverse-security/go,josharian/go,codestation/go