restore currently disabled tests for MSVC++

[dietmarkuehl]

Two tests caused grief in the CI when building on MSVC++. Currently, I don't have access to a Windows machine to debug the issues and they are just disabled:

1. In exec-just.test.cpp the test using an allocator: test_just_allocator. Somehow it seems a call to make_obj_using_allocator is blocking. That is only happening for the debug build.
2. In exec-then.test.cpp a static_assert failed in test_has (the invocation from test_has<true>(test_std::then, test_std::just(), [] {});. I don't know what's going wrong here. That is only happening for the release build.