Exclude benchmark apps from builds (#7171)

build/repo.props

src/Mvc/benchmarkapps/Directory.Build.props

src/Mvc/benchmarkapps/Directory.Build.targets

.../Microsoft.AspNetCore.Mvc.FunctionalTests/BasicApiTest.cs

...icrosoft.AspNetCore.Mvc.FunctionalTests/BasicViewsTest.cs

...ore.Mvc.FunctionalTests/Infrastructure/BasicApiFixture.cs

...e.Mvc.FunctionalTests/Infrastructure/BasicViewsFixture.cs

...onalTests/Microsoft.AspNetCore.Mvc.FunctionalTests.csproj

...ers/Kestrel/perf/PlatformBenchmarks/Directory.Build.props

...s/Kestrel/perf/PlatformBenchmarks/Directory.Build.targets

...Kestrel/perf/PlatformBenchmarks/PlatformBenchmarks.csproj

src/SignalR/perf/benchmarkapps/Directory.Build.props

src/SignalR/perf/benchmarkapps/Directory.Build.targets