RemoteExecutor support on platforms which cannot spawn a process

- [ ] This issue is blocking
- [ ] This issue is causing unreasonable pain

RemoteExecutor should have a special "no-op" mode to be able to run hundreds of the existing tests on platforms where you can't spawn processes (e.g. iOS, tvOS, etc). It looks like there are more than 400 RemoteExecutor-based tests in dotnet/runtime and disabling them all for iOS one by one can be painful. We had it in mono/mono: https://github.com/mono/mono/blob/master/mcs/class/test-helpers/RemoteExecutorTestBase.Mobile.cs#L19