test/integration: "Error checking" test is broken

[hyangah]

The test started to fail after https://go-review.googlesource.com/c/vscode-go/+/405551 (rearranging diagnostics collection passing) with go1.17 or older versions.

But our kokoro CI failed to catch it because our kokoro CI runs with go1.18 and the test was disabled to workaround while waiting for staticcheck go1.18 support.

(what is worse is we aren't monitoring github long tests workflow failures)

[gopherbot]

Change https://go.dev/cl/411104 mentions this issue: test/integration: reenable staticcheck and fix "Error checking" test