Reduce the run time of tests

Some tests take a long time to run, making them harder to use for regression testing. We should look for opportunities to reduce the execution time while retaining enough coverage.

Start by profiling the OpenCL conformance test suite to identify the tests responsible for the long runtime, then review their coverage and regression-testing role. The work is done when execution time is reduced while the necessary coverage is retained, with measurements showing the improvement.

Written by the indexing model from the issue text.