test: test_fs error message

For non-root users mkfs.vfat is not in the search path at least on Debian.
Hence when running 'make tests' a message indicates that file system tests
have been skipped:

SKIPPED [13] test/py/tests/test_fs/conftest.py:340: Setup failed for
filesystem: fat16

This message is not really helpful as the executed program is not
indicated. Provide a more complete message like

SKIPPED [13] test/py/tests/test_fs/conftest.py:340: Setup failed for
filesystem: fat16.
Command 'mkfs.vfat -F 16 build-sandbox/persistent-data/3GB.fat16.img'
returned non-zero exit status 127.
Signed-off-by: NHeinrich Schuchardt <xypron.glpk@gmx.de>

test/py/tests/test_fs/conftest.py

@@ -335,8 +335,9 @@ def fs_obj_basic(request, u_boot_config):
         md5val.append(out.split()[0])
 
         umount_fs(mount_dir)
-    except CalledProcessError:
-        pytest.skip('Setup failed for filesystem: ' + fs_type)
+    except CalledProcessError as err:
+        pytest.skip('Setup failed for filesystem: ' + fs_type + \
+            '. {}'.format(err))
         return
     else:
         yield [fs_ubtype, fs_img, md5val]