fstests: modify user name beginning with non-digit

fstests: modify user name beginning with non-digit

openSUSE and SLE don't support username begin with digit, so it will
skip test generic/597 and generic/598 by lack of 123456-fsgqa user.
generic/597 and 598 are not test username begin with digit on purpose
(different with generic/381). It's will be helpful to use an username
begin with non-digit in this case.

Signed-off-by: Sun Yong <yosun@suse.com>
Reviewed-by: Petr Vorel <pvorel@suse.cz>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>