Fix System.Configuration.ConfigurationManager.Tests on Android and reenable on non...

Fix System.Configuration.ConfigurationManager.Tests on Android and reenable on non-Windows platforms (#56558)

The test assembly got changed to `$(NetCoreAppCurrent)-windows` in https://github.com/dotnet/runtime/commit/809a06f45161ae686a06b9e9ccc2f45097b91657#diff-4d639cb37fe53cdeae4262c1f5be7936cdd81e3b4f256f9c59ad02ec69b300d9R7 but when talking to Viktor we're not sure why, probably a mistake.

Target `$(NetCoreAppCurrent)` instead so it runs on all platforms and fix a test issue that occurs on Android due to BaseDirectory not having a trailing slash. Fixes https://github.com/dotnet/runtime/issues/37071
Incidentally, that's likely also the reason why the .csproj set `TestDisableAppDomain` so we can remove that as well.

Also replace `TestDisableParallelization` with the assembly attribute equivalent which is what we use everywhere else.