C++ test false positive errors with non-Linux hosts

[feltech]

What

C++ tests fail on non Linux hosts

Why

MacOS: the test "Deprecated specification causes deprecation compiler warning" fails, i.e. the CTest regex check of the build output cannot find the expected warning. Perhaps it's being optimised out?

Windows: unrecognised compiler flag -Wno-deprecated-declarations. We set this flag to prevent the main tests from failing (due to -Werror "warnings-as-error"). However, /wd4996 is the correct flag on Windows.

Unfortunately, enabling MacOS and Windows C++ tests is tricky, since we currently rely on a Docker build of OpenAssetIO for all C++ tests, and therefore only test Ubuntu. See OpenAssetIO/OpenAssetIO#1235 (and its parent issue).

However, the integration tests in the upstream OpenAssetIO repo do run on multiple OSes, which is what flagged these issues.

[feltech]

Had to disable a CI step on non-Ubuntu in the upstream OpenAssetIO repo (see OpenAssetIO/OpenAssetIO#1481) - we should re-enable that test when this is resolved.

[feltech]

Ideally, we'd take this opportunity to make use of the build artifacts that we now upload from the OpenAssetIO repo's CI (OpenAssetIO/OpenAssetIO#1201), to allow us to matrix the tests across all three platforms, and avoid having to build OpenAssetIO anew every time we trigger CI (i.e. OpenAssetIO/OpenAssetIO#1235)