Do not fail pipeline if optional tests fail

The VSTest task now fails if no assemblies match the test filter.
Since we don't have any Windows-only test assemblies currently, we can
just continue on error for this task.

Author: mjcheetham

.azure-pipelines/templates/windows/compile.yml

@@ -46,3 +46,4 @@ steps:
     configuration: 'Windows$(configuration)'
     otherConsoleOptions: '/Framework:.NETFramework,Version=v4.6.1'
     testRunTitle: 'Unit tests - helpers (Windows)'
+  continueOnError: true