[release/v1.2] service-mesh: test readiness with exec probe

[edgelessci]

Backport of #1142 to release/v1.2.

Original description:

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The startup probe for the mesh proxy is redirected onto the same port, causing Envoy to open connections to itself recursively. Unfortunately, this is not visible in the Envoy logs - I only discovered this condition while working on a custom proxy implementation.

Things that are still unclear:

• There is some race condition that causes the redirection loop to only occur ~50% of the time.
• Switching on debug logging makes this condition much rarer.
• Exempting the TPROXY ports with iptables should make the SO_ORIGINAL_DST lookup fail, but the loop happens nonetheless.
• The issue reproduces with opening and holding a single connection, while opening another, but not with just one connection.

Tests:

• https://github.com/edgelesssys/contrast/actions?query=branch%3Aburgerdev%2Fmesh-probe+event%3Aworkflow_dispatch++