Migrate legacy-only tests to net10.0 ahead of the tool-only 12.0

[fpellet]

Paket 12.0 drops paket.exe, the bootstrapper, Mono and the net461 target framework (see #4300). Some tests only did their real work on net461 today, sometimes in no CI job at all, while the code they cover is shared with the .NET tool. This PR moves them to net10.0 before anything is removed, so the CI still compares both frameworks while they migrate. The removal itself comes in a follow-up PR.

Bug found on the way: no_proxy was ignored by the .NET tool

The netstandard build replaced WebProxy with a hand-written IWebProxy:

• its IsBypassed compared the whole URL with the regex patterns built from no_proxy, so no entry ever matched;
• its default proxy ignored the system settings, unlike paket.exe.

The tests that would have caught it skipped exactly these assertions on net10.0 (//TODO readd check). WebProxy and WebRequest.GetSystemWebProxy are part of netstandard2.0, so both targets now use them. This is the only change outside the tests, with a release note.

What the .NET tool now does with no_proxy depends on where the proxy comes from:

• HTTP_PROXY / HTTPS_PROXY: Paket builds the proxy and applies no_proxy itself, as curl does. An entry matches the host and its subdomains, * alone matches every host, and a matching host is reached directly.
• ALL_PROXY only: Paket hands the request to the .NET system proxy, which reads ALL_PROXY and applies its own no_proxy rules. An entry matches that exact host, a leading . matches its subdomains, and * is not supported.

Tests now running on net10.0

• Resolver timeout and cancellation tests: TimeoutAttribute works on netstandard NUnit, so NO_UNIT_TIMEOUTATTRIBUTE is gone.
• Credential round-trip tests now run on every OS. Outside Windows the encryption uses AES, not ProtectedData, so the ignore was stale.
• Integration tests that only ran on net461: paket outdated should optionally evaluate version constraints #183 outdated flags, Missing transitive dependencies after paket update #1190, Makes conflicts fail faster #1174, Paket failed with UnauthorizedAccessException writing to MyProject.paket.references.cached #3410 and 3.1.7: "paket install --log-file" blows up right away with IndexOutOfRangeException #1743.
  • [<Platform>] is not usable here: on net10.0 with NUnit 3.12 it silently drops the discovery of the whole module, which I checked on UtilsSpecs. So these tests check the OS at run time instead, and NO_UNIT_PLATFORMATTRIBUTE stays for the unit tests.
  • paket outdated should optionally evaluate version constraints #183 had Platform "Mono" and ran in no CI job.
• New test for paket init on net10.0. The command was only covered through the bootstrapper test.
• Three install XML tests were marked flaky on .NET Core only, since 2018. They pass reliably now, so they move to the main job instead of ending up flaky-only once net461 is gone.
• Dead defines removed: TESTSUITE_KNOWN_FAILURE_DOTNETCORE_3005, FAKE_NETSTANDARD_API and WEBPROXY_NETSTANDARD.

New coverage: how Paket.Restore.targets finds the .NET tool

Every integration test pointed Paket.Restore.targets to the build output through PaketExePath. As a result, the branches that find a real installation had no coverage, and they are the only ones left in 12.0. DotnetToolSpecs installs the package the build packs and restores a project without PaketExePath, with paket installed in each of these ways:

• as a local tool of the manifest;
• with --tool-path .paket;
• in a folder on the PATH, the way a global tool is.

How it stays isolated:

• The install only uses temp/ and a packages folder of the scenario, because the global one may hold the nuget.org package of the same version.
• The scenario gets its own empty tool manifest, so dotnet never walks up to the repository's manifest.

The NuGet build target now runs before the integration tests. The tests are ignored on a local dotnet test when no package was packed, and fail on CI, where the package is expected.

Not migrated

Everything in Paket.Bootstrapper.Tests is bootstrapper-only. Its proxy tests were the only overlap, and UtilsSpecs covers the same cases.

Issue: #4300