runtime: really wait for goroutines in testSemaHandoff
runtime: really wait for goroutines in testSemaHandoff

The code has a comment saying that it waited for the goroutines,
but it didn't actually do so.

Change-Id: Icaeb40613711053a9f443cc34143835560427dda
Reviewed-on: https://go-review.googlesource.com/c/go/+/219277
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

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