test: Don't start server until all configured

[paul1r]

What this PR does / why we need it:
Before the fix:

WARNING: DATA RACE
Read at 0x00c0003ae150 by goroutine 151:
  net/http.(*conn).setState()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/net/http/server.go:1810 +0x120
  net/http.(*Server).Serve()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/net/http/server.go:3085 +0x594
  net/http/httptest.(*Server).goServe.func1()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/net/http/httptest/server.go:310 +0xb8

Previous write at 0x00c0003ae150 by goroutine 150:
  github.com/grafana/loki/pkg/storage/chunk/client/gcp.fakeSleepingServer()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/client/gcp/gcs_object_client_test.go:253 +0x140
  github.com/grafana/loki/pkg/storage/chunk/client/gcp.TestTCPErrs.func1()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/client/gcp/gcs_object_client_test.go:209 +0xb8
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1595 +0x1b0
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x40

Goroutine 151 (running) created at:
  net/http/httptest.(*Server).goServe()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/net/http/httptest/server.go:308 +0x98
  net/http/httptest.(*Server).Start()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/net/http/httptest/server.go:134 +0x31c
  net/http/httptest.NewServer()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/net/http/httptest/server.go:107 +0x1a0
  github.com/grafana/loki/pkg/storage/chunk/client/gcp.fakeSleepingServer()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/client/gcp/gcs_object_client_test.go:249 +0x78
  github.com/grafana/loki/pkg/storage/chunk/client/gcp.TestTCPErrs.func1()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/client/gcp/gcs_object_client_test.go:209 +0xb8
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1595 +0x1b0
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x40

Goroutine 150 (running) created at:
  testing.(*T).Run()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x5e8
  github.com/grafana/loki/pkg/storage/chunk/client/gcp.TestTCPErrs()
      /Users/progers/dev/src/github.com/grafana/loki/pkg/storage/chunk/client/gcp/gcs_object_client_test.go:208 +0x8c
  testing.tRunner()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1595 +0x1b0
  testing.(*T).Run.func1()
      /opt/homebrew/Cellar/go/1.21.6/libexec/src/testing/testing.go:1648 +0x40
==================
--- FAIL: TestTCPErrs (0.10s)
    --- FAIL: TestTCPErrs/request_took_longer_than_client_timeout,_not_retryable (0.01s)
        testing.go:1465: race detected during execution of test
    testing.go:1465: race detected during execution of test
FAIL
FAIL	github.com/grafana/loki/pkg/storage/chunk/client/gcp	1.754s
FAIL

After the fix:

go test -race -count=50 . -run TestTCPErrs/request_took_longer_than_client_timeout,_not_retryable
ok  	github.com/grafana/loki/pkg/storage/chunk/client/gcp	2.711s

Which issue(s) this PR fixes:
Relates to: #8586

Special notes for your reviewer:

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• CHANGELOG.md updated
  • If the change is worth mentioning in the release notes, add add-to-release-notes label
• Changes that require user attention or interaction to upgrade are documented in docs/sources/setup/upgrade/_index.md
• For Helm chart changes bump the Helm chart version in production/helm/loki/Chart.yaml and update production/helm/loki/CHANGELOG.md and production/helm/loki/README.md. Example PR
• If the change is deprecating or removing a configuration option, update the deprecated-config.yaml and deleted-config.yaml files respectively in the tools/deprecated-config-checker directory. Example PR