Found while developing #2376.
Parts of "test_opencl_c_versions" were intending to build programs for
different OpenCL C versions, but unfortunately the helper function the
test was calling only created the program and did not build it. Switch
to a helper function that builds the program, instead.
* Skip compiler unload/reload tests if compiler is not available
Note that tests that use the compiler helper functions but won't work without a
compiler are in the `subtests_to_skip_with_offline_compiler` list. I didn't do
that here because they tests directly use `clBuildProgram` etc directly,
because they're specifically testing it's behaviour in edge-cases.
* Change type of status variable
* add test_compiler test for OpenCL C versions
* add checks for required OpenCL C versions
* bugfix: only OpenCL C 2.0 is required for OpenCL 2.1 and 2.2
* pass reference to feature struct vs. pointer
* address review comments regarding C++ identifiers and 3d image writes
