[tests] Fix the build for link sdk tests on .NET/macOS.

[rolfbjarne]

Fixes these errors:

xamarin-macios/tests/linker/ios/link sdk/LinkSdkRegressionTest.cs(1140,41): error CS0246: The type or namespace name 'NSApplication' could not be found (are you missing a using directive or an assembly reference?)
xamarin-macios/tests/linker/ios/link sdk/LinkSdkRegressionTest.cs(1156,4): error CS0103: The name 'NSApplication' does not exist in the current context
xamarin-macios/tests/linker/ios/link sdk/LinkSdkRegressionTest.cs(1157,4): error CS0103: The name 'NSApplication' does not exist in the current context

[vs-mobiletools-engineering-service2]

❌ [PR Build] Tests failed on Build ❌

Tests failed on Build.

• Azure DevOps
• Html Report (VSDrops)

API diff

✅ API Diff from stable

View API diff

• iOS vsdrops gist
• macOS vsdrops gist
• macCat vsdrops gist
• macCatiOS gist
• tvOS vsdrops gist
• watchOS vsdrops gist

API & Generator diff

✅ API Diff (from PR only) (no change)
✅ Generator Diff (only version changes)

GitHub pages

Results can be found in the following github pages (it might take some time to publish):

• Test results
• API diff
• API & Generator diff

Test results

1 tests failed, 140 tests passed.

Failed tests

• monotouch-test/Mac Catalyst/Debug [dotnet]: Failed (Tests run: 2638 Passed: 2489 Inconclusive: 35 Failed: 1 Ignored: 148)

Pipeline on Agent XAMBOT-1099.BigSur'
Merge 03109b6 into db98082

[rolfbjarne]

Test failure is unrelated (https://github.com/xamarin/maccore/issues/2443)