Add tests/Testing.cpp and create osimTesting target

[nickbianco]

Part 1 of 4 changes related to #3555 and #4393.

These changes were originally written all together on a different branch that was supposed to only add Testing.cpp and some minor changes, but eventually ballooned as I started to pull on threads. Therefore, I used Claude to split the changes on the branch onto four different branches and created a stack of PRs , a new feature offered by the GitHub CLI (e.g., gh stack) that I wanted to try out.

Brief summary of changes

• Moves all non-macro test helpers in Testing.h inside the new OpenSim::Testing namespace (except for CHECK_STORAGE_AGAINST_STANDARD which gets converted in part 2).
• Moves the implementation of all (non-templatized) test helpers to the new file Testing.cpp.
• Packages the test helper functions and macros into the new build target osimTesting.

Testing I've completed

Ran tests locally; CI.

Looking for feedback on...

CHANGELOG.md (choose one)

• no need to update because...updating internal test layout.

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This change is 

[adamkewley]

@nickbianco looks like a sensible cleanup 👍