Test a project that targets both net10.0 and net11.0 with runtime async #146

Add at least one test that builds a single project targeting both net10.0 and net11.0, with runtime async enabled on the net11.0 target.

Runtime async makes two IL shapes coexist in one build. The net10.0 target compiles async methods to classic state machines. The net11.0 target compiles them to runtime-async methods, which have no state machine. A matrix that builds one target framework per project cannot detect a regression in how PostSharp chooses between the two shapes.

Runtime async is not enabled for user code by default in .NET 11. The net11.0 target therefore needs <Features>runtime-async=on</Features>. Without it, both targets produce classic state machines and the test cannot detect anything. See #103.

The expected result of the test depends on the design in #103, which defines what PostSharp does with a runtime-async method.

Extracted from #88.

— Claude for Gael