endtoend: fix race when closing vtgate

[vmg]

Description

This fixes a race we've found that can sometimes get the endtoend tests stuck permanently during shutdown. cc @dbussink

Related Issue(s)

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[vitess-bot]

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[deepthi]

This has been a long-standing issue, thank you for fixing it!
Not sure why you've got a go.sum diff, that change is already on main.