[WinUI OSS] .gitignore is missing build/test artifacts, e.g. TestPayload/ shows up as untracked

[gpaskaleva-msft]

Description

The repo's .gitignore is out of sync with the artifacts our own scripts and
pipelines produce. After a normal init.cmd + build + test cycle, generated output
shows up as candidates to commit — most visibly the TestPayload/ folder created by
test\CreateTestPayload.ps1, and dxaml\test\infra\taefhostappnetcore\WinRT.Host.runtimeconfig.json
written by scripts\init\SetupDotNetFiles.cmd on every init.

The following in-repo generated artifacts are unignored here:

• TestPayload* — test\CreateTestPayload.ps1
• dxaml/test/infra/taefhostappnetcore/WinRT.Host.runtimeconfig.json — SetupDotNetFiles.cmd
• .buildtools/ — scripts\init\Initialize-InstallMSBuild.ps1 (repo-local VS install)
• MSBuildCache/, MSBuildCacheLogs/ — eng\projectcaching.props / Build.cmd
• msbuild-install-logs/ — WinUI-BuildVariables.yml
• .sln.metaproj, .ProjectImports.zip, build.err, build.wrn
• *.coverage, *.coveragexml
• *.iobj, *.ipdb, *.ndf, *.jfm, *.e2e, *.sap, *.vbw, *.nvuser, CppProperties.json
• .idea/, *.sln.iml, pycache/, *.pyc
• perf/results, perf/scenarios//{x86,x64}/

Risk: contributors accidentally commit build output, and git status noise hides
real changes.