Prefix to insert before the executables for the 'test' and 'run' commands. If the value is 'foo -bar', and the execution command line is 'test_binary - baz', then the final command line is 'foo -bar test_binary -baz'.This can also be a label to an executable target. Some examples are: 'valgrind', 'strace', 'strace -c', 'valgrind --quiet --num-callers=20', '//package: target', '//package:target --options'.