Headers missing when building loader is turned off

If the tests are built with building of the loader off, the openxr headers are never included and the build fails. This is because the header dependency is only made explicit in the loader build script. A possible solution is to make that dependency explicit for all tests: https://github.com/KhronosGroup/OpenXR-SDK-Source/pull/439

This was detected while using the latest release with the Quest SDK:
https://github.com/pfontain/OpenXR-SDK-Source/commit/2e7def6b72a98c293ff1b4d8d7e767060f60bdc3

Start by inspecting the loader build script and the test build configuration, focusing on where the OpenXR header dependency is declared. Reproduce a test build with loader building disabled, then verify that all tests still receive the headers and the build completes successfully; PR #439 provides the suggested direction.

Written by the indexing model from the issue text.