Allow the user to turn off tests with -DBUILD_TESTING=OFF

[haampie]

Currently tests are always built, this PR allows the user to turn off tests through cmake -DBUILD_TESTING=OFF, which is pretty much a standardized cmake variable from CTest.

The reason to introduce this is because of a circular dependency in hip and comgr's tests:

Say I have a system install of hip version 3.7.0 and I use the spack package manager to build hip 3.10.0, it will first have to build comgr 3.10.0, which by default builds tests, which will make it look for hip and it'll find the system version 3.7.0. This is undesired and there's little we can do about it, except for just disabling the tests.

Solves https://github.com/spack/spack/issues/20132

[haampie]

Do you accept PRs here @slinder1?

[lamb-j]

https://github.com/RadeonOpenCompute/ROCm-CompilerSupport/commit/5b684e557c67ef1a8482117c462ed25ac4a805d6