There are some basic tests that can be run qith qemu. You can run all the tests with

    $ make check

or selectively with:

    $ make run-hello

Also, you can debug the existing tests, or a new one, by starting on one shell

    $ make debug-hello

and on another shell you can attach with gdb, load the symbols of the
bootstrap module and break on its _start():

    $ gdb gnumach
    ...
    (gdb) target remote :1234
    ...
    (gdb) b setup_main
    Breakpoint 11 at 0xffffffff81019d60: file ../kern/startup.c, line 98.
    (gdb) c
    Continuing.

    Breakpoint 11, setup_main () at ../kern/startup.c:98
    98		cninit();
    (gdb) add-symbol-file ../gnumach/build-64/module-task
    Reading symbols from ../gnumach/build-64/module-task...
    (gdb) b _start
    Breakpoint 12 at 0x40324a: _start. (2 locations)
    (gdb) c
    Continuing.

    Breakpoint 12, _start () at ../tests/testlib.c:96
    96	{
    (gdb)