added some tests

Added tests for `context_error` and `unsupported_extension`.
i'd usually name these something like `exception.context_error` based on where they are in the source tree (`boost/compute/exception/`). this allows you to use ctest to run only specific sets of test for specific parts of the code, which can be useful when testing new changes. for example, currently you can run all of the exception tests by doing `ctest -R exception.*` (see the `exception.opencl_error` line below).