Duplicate test cases in runtime install tests cause 24 skipped tests per run #701

Problem

GivenThatIWantToInstallDotnetFromAScript generates duplicate test cases for several runtime install methods, resulting in 24 "Skipping test case with duplicate ID" warnings per test run. These are full download-and-install integration tests, so the duplicates that do run are wasting CI time, and the skip warnings in the output create noise that can be mistaken for a real problem.

Cause

InstallRuntimeFromChannelTestCases iterates both _runtimeBranches and _channels to generate test data. Several entries in _runtimeBranches are not actually branches — they are channels that also appear in _channels with the same quality:

When Quality.None is passed through GetQualityOptionsFromFlags, it yields nothing, and .DefaultIfEmpty() produces a single null — so both lists generate identical test parameters (channel, null, versionRegex).

This affects 4 test methods (WhenInstallingDotnetRuntime, WhenInstallingAspNetCoreRuntime, WhenInstallingWindowsdesktopRuntime, WhenInstallingDotnetRuntimeWithFeedCredential), producing 24 duplicate test cases that xUnit skips.

Impact

• Confusing output — 24 "Skipping test case with duplicate ID" warnings look like something is wrong
• Wasted CI time — the non-skipped half of each duplicate pair still runs a full SDK/runtime download