Fix: a data race condition after a timeout on an `assert.Eventually` call.

[kcross-ctoken]

Summary

Fixes a data race when using assert.Eventually so the variables can be safely used after a timeout

This was found in my test code when the eventually timed out and I was using something set in the closure. It required the --race go test parameter set.

Changes

added a wait group to wait for the end of the go routine call in assert.Eventually

[brackendawson]

#1657 would resolve the issue here I believe.