[bcl] Fix assembly identity of nunit/nunitlite (#12725)
* [bcl] Fix assembly identity of nunit/nunitlite
The new strong name key added in https://github.com/mono/mono/commit/
acaa73b7026897f7e83a101a9bc331476b5ed169
changed the assembly identity which we can't do for shipping libraries.
Given that the old .snk is invalid (see https://github.com/dotnet/roslyn/issues/32502),
we just sign with the extracted public key and /publicsign.
For MCS_MODE builds we instead use the mono.snk since mcs doesn't support /publicsign
and we don't really care about having the correct identity there.
* [csproj] Update project files