Compile tests without running

[hwchen]

Currently, using compile-test, the testrun executable will be compiled and then automatically run. However, there may be some cases when it's not desirable to immediately run, such as compiling test executable for use with debugger.

This can be implemented with a flag like --no-run. --build-only has also been suggested. As a point of posible confusion, compile-only subcommand already exists, which compiles but does not link.

Not sure further discussion is needed, but as an initial thought, checking for this flag can just be placed after the check for compile-test, compile-bench, etc, and just switch off the run. Let me know if there's other considerations.

[lerno]

Note that --testing and --benchmark already includes the test but without the actual test/bench runner created as the main.