misc/cgo/test: further reduce likeliness of hang in Test9400

As suggested by #49680, a GC could be in-progress when we
disable GC. Force a GC after we pause to ensure we don't
hang in this case.

For #49695

Change-Id: I4fc4c06ef2ac174217c3dcf7d58c7669226e2d24
Reviewed-on: https://go-review.googlesource.com/c/go/+/367874


Run-TryBot: Paul Murphy <murp@ibm.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Trust: Paul Murphy <murp@ibm.com>