[E2E] Diagnostics collection from Kibana

[pierrehilbert]

Create a E2E test to validate:

1. We are able to collect diagnostics from Kibana
2. Diagnostic contains what it should

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[jlind23]

I don't think this issue is required anymore:

• There is a kibana test here: https://github.com/elastic/kibana/blob/main/x-pack/plugins/fleet/server/services/agents/request_diagnostics.test.ts
• There is an Elastic Agent test here:

  elastic-agent/testing/integration/diagnostics_test.go

  Line 228 in e984ed2

  func testDiagnosticsFactory(t *testing.T, compSetup map[string]integrationtest.ComponentState, diagFiles []string, diagCompFiles []string, fix *integrationtest.Fixture, cmd []string) func(ctx context.Context) error {