Fix a bunch of annoying compiler warnings in tests

Reference issue

What does this implement/fix?

Should substantially reduce the number of warnings emitted during the MR smoketests, specifically visitor.cpp, adjoint.cpp, and array_cwise.cpp. This is mostly due to narrowing conversions -- nothing material.

Job buildsmoketests:x86-64:linux:clang-10:cxx11-on log:

Before: 2189 lines

After : 1193 lines

The only warnings are associated with the deprecation of pow, which is intentional.

Additional information