The get_device_il_version_string() function throws an exception if the
device does not support the CL_DEVICE_IL_VERSION query, so don't call
this unless the version is recent enough.
* 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
The called function (get_device_info_string) subtracts 1 from
length before returning it. Hence, to check for an empty string
(that the spec requires the implementation to return when
CL_DEVICE_IL_VERSION is not supported) the check should be modified
to check for empty and not length == 1
