fix tests when using busybox

[joshrabinowitz]

There's a few issues we see testing with busybox (such as on Alpine Linux without coreutils installed)

** we get an error at tests/test_hide.bats line 86:

[ " $secret_perm" = "$file_perm" ] failed
ps: unrecognized option: w

There's multiple issues here: umask might make our permissions test fail; busybox's ps and stat are different than coreutils's

See also #477 which tries to deal with more of these issues.