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: Heinrich Schuchardt <xypron.glpk@gmx.de>