test: Add a knob to disable admin server in IntegrationTestServer

[fredyw]

This PR adds a knob to explicitly disable the admin server in the IntegrationTestServer.

Risk Level: low (for testing only) Testing: existing tests Docs Changes: n/a Release Notes: n/a Platform Specific Features: n/a

[repokitteh-read-only[bot]]

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Please mark your PR as ready when you want it to be reviewed!

:cat:

Caused by: https://github.com/envoyproxy/envoy/pull/36684 was opened by fredyw.

see: more, trace.

[fredyw]

/assign @alyssawilk