Start inferior before running test listing Ada exceptions.

Start inferior before running test listing Ada exceptions.

This patch fixes some spurious failures when the inferior is linked
against the shared version of libgnat by default, as appears to be
the case on many GNU/Linux distributions.  When that happens, we have
to start the program in order to ensure that the GNAT runtime is
mapped to memory, in order for us to find the standard exceptions
(defined within the runtime).  Otherwise, they will not be shown,
as expected, by the debugger.

gdb/testsuite/ChangeLog:

        * gdb.ada/info_exc.exp: Start inferior before starting
        the "info exceptions" tests.
        * gdb.ada/mi_exc_info.exp: Start inferior before starting
        the "-info-ada-exceptions" tests.