Add a test spouse for server context

Makefile

build.yaml

include/grpc++/impl/codegen/server_context.h

include/grpc++/test/server_context_test_spouse.h

src/cpp/test/server_context_test_spouse.cc

tools/run_tests/sources_and_headers.json

vsprojects/vcxproj/test/grpc++_test/grpc++_test.vcxproj

...ects/vcxproj/test/grpc++_test/grpc++_test.vcxproj.filters