Fix go 1.15 int-to-string conversion warning in tests

[aoldershaw]

What does this PR accomplish?

Bug Fix | Feature | Documentation

Go 1.15 added automated vetting in tests for this particular case, and it's causing our unit tests to fail.

https://golang.org/doc/go1.15#vet

Changes proposed by this PR:

Properly convert integers to strings

Notes to reviewer:

Release Note

Contributor Checklist

• Followed Code of conduct, Contributing Guide & avoided Anti-patterns
• Signed all commits
• Added tests (Unit and/or Integration)
• Updated Documentation
• Added release note (Optional)

Reviewer Checklist

• Code reviewed
• Tests reviewed
• Documentation reviewed
• Release notes reviewed
• PR acceptance performed
• New config flags added? Ensure that they are added to the
  BOSH and
  Helm packaging; otherwise, ignored for
  the integration
  tests
  (for example, if they are Garden configs that are not displayed in the
  --help text).

[taylorsilva]

LGTM!

[chenbh]

There's some other stuff go vet ./... emits that we should probably fix:

# github.com/concourse/concourse/atc/metric/emitter
atc/metric/emitter/newrelic.go:216:8: using resp before checking for errors
# github.com/concourse/concourse/atc/metric/emitter_test
atc/metric/emitter/prometheus_test.go:202:9: using res before checking for errors
# github.com/concourse/concourse/atc/worker_test
atc/worker/choose_task_worker_test.go:55:9: the cancel function returned by context.WithCancel should be called, not discarded, to avoid a context leak
atc/worker/client_test.go:382:9: the cancel function returned by context.WithCancel should be called, not discarded, to avoid a context leak
# github.com/concourse/concourse/worker/runtime/integration/sample
worker/runtime/integration/sample/main.go:37:3: Fatal call has possible formatting directive %s
# github.com/concourse/concourse/topgun/k8s_test
topgun/k8s/dns_proxy_test.go:65:14: result of fmt.Errorf call not used

[taylorsilva]

@chenbh maybe in a separate issue and PR? I get the same errors from go vet on go 1.14. Those errors are also currently not blocking every PR that's currently in flight 😓

[aoldershaw]

We should also probably be running go vet in CI - might be worth creating an issue for that. Going to merge this now to unblock PRs, though - but don't want to lose track of this conversation!