Fix OpenCL test failures on Solaris.

Since nobody currently makes an OpenCL driver for Solaris, all the
tests used to fail, which told us nothing. This commit adds code
to check whether OpenCL is unavailable and the OS is Solaris, in
which case the test contents are skipped. If an OpenCL driver ever
appears for Solaris, or if we start testing on another platform
with no OpenCL driver, there's now one single place to add or remove
checks that will allow for this.