test(e2e): always tear down DDA before next subtest to avoid races

[tbavelier]

Summary

• Adds an applyDDA helper to TestGenericK8s that does a WithoutDDA pre-step before applying each new DatadogAgent, and routes every DDA-applying subtest through it.
• Fixes the K8s 1.19-only flake on KSM_check_works_cluster_check_runner where the new node-agent DaemonSet pod would sit in FailedMount on its auto-generated <sa>-token-<rand> Secret because the legacy SA-token controller (pre-TokenRequest, pre-1.20) couldn't keep up with the SA delete+create churn from a direct DDA swap.
• Collapses the inline two-step WithoutDDA boilerplate that APM and DSD UDP subtests already had (net −5 lines).

The same race is latent on every other subtest that swaps DDAs; doing the cleanup uniformly removes that exposure (DSD UDS, Autodiscovery, Logs were unprotected before). On K8s ≥1.20 the cleanup step is essentially a no-op for the SA-token path, but still useful for hostPort rebinding.

Test plan

• make K8S_VERSION=1.19 e2e-tests passes without retries
• CI matrix passes on the other supported K8s versions

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.92%. Comparing base (c08166e) to head (fe6a71a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2954   +/-   ##
=======================================
  Coverage   40.92%   40.92%           
=======================================
  Files         324      324           
  Lines       28750    28750           
=======================================
  Hits        11767    11767           
  Misses      16129    16129           
  Partials      854      854           

Flag	Coverage Δ
unittests	40.92% <ø> (ø)

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[datadog-prod-us1-6]

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 41.04% (+0.00%)

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