[CLI] Add qmk list_keyboards (#6927)

`list_keyboards` replicates the `make list-keyboards` by globbing for all paths
that include `rules.mk` and then removing the paths that include `keymaps`.

This basis of this cli command could be reused in the future as a util, but is
not done so here since this would be the only place that would use it currently

Resolves #6911

lib/python/qmk/tests/test_cli_commands.py

@@ -37,3 +37,11 @@ def test_pyformat():
     result = check_subcommand('pyformat')
     assert result.returncode == 0
     assert 'Successfully formatted the python code' in result.stderr
+
+
+def test_list_keyboards():
+    result = check_subcommand('list_keyboards')
+    assert result.returncode == 0
+    # check to see if a known keyboard is returned
+    # this will fail if handwired/onekey/pytest is removed
+    assert 'handwired/onekey/pytest' in result.stdout