Improving VHO tests in an attempt to investigate a bug

[ryanrolds]

Description

• Changed the header names because the test output showed Nginx responding with X-Transfer-Encoding: chunked, which means Content-Length was never on the response
• Order the steps in each test to more strongly assert that conflict resolution and merging didn't affect earlier applied resources
• Moved the curl check at the end so that it's asserting against a confirmed desired state
• Renamed some things to better clarify what is happening in the test
• Removed the manifest list and added cleanup callback to give each finer grained and a clear picture of the test

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

[solo-changelog-bot]

Issues linked to changelog:
k8sgateway#10310

[github-actions]

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[sam-heilbron]

Conceptually the changes look good to me! I'd like to run them locally to better understand them, but leaving an approval as I think this is good to merge

[sam-heilbron]

nit: Since the ContainHeaderKeys matcher supports a list, any reason we don't combine these into a single assertion?