[cross-project-tests] Make GDB version string parsing more robust

Follow up to D118468 (5257efdc).

When built from source, gdb's version string looks like this:

    GNU gdb (GDB) 9.2
    ...

But for installed versions it looks different. E.g.

    GNU gdb (Ubuntu 9.2-0ubuntu1~20.04.1) 9.2
    ...

Use a regex rather than str.parition in the version string parsing in order to
handle this case too.