x/tools/gopls: test cleanup errors during shutdown with ERROR_SHARING_VIOLATION on Windows #53819
Description
Following the recent change to await proper shutdown of the regtest environment (CL 416594), we get one new flake on windows:
--- FAIL: TestRegenerateCgo (7.42s)
--- FAIL: TestRegenerateCgo/experimental (4.02s)
runner.go:322: closing the sandbox: error(s) cleaning sandbox: cleaning modcache: <nil>; removing files: remove C:\Users\gopher\AppData\Local\Temp\gopls-regtest-2462642078\TestRegenerateCgo\experimental\work: The process cannot access the file because it is being used by another process.
https://build.golang.org/log/27834526d9f9152f3bc200e902c97d8a73d55452
https://build.golang.org/log/b4e52dbee66f77feae2f7fa17e2981087e7fbbcb
If we've learned anything from investigating these flakes, it is that they are often (if not usually) actual races in gopls. Unfortunately the goroutine dump at test failure point does not show anything interesting. It is possible that problematic goroutines had exited before the dump.