Rework component testing support to make test updates easier

[elliottt]

Add a macro for generating the core wasm and component versions of a test, and
modify the .adapt_component builder function on Test to accept a bool. This
simplifies modifying existing tests to support components by doing the work of
abstracting out the body in a macro.

[jameysharp]

Yup, this strikes me as a net positive for test clarity, in spite of my general preference to avoid macros. 👍