tst/run.sh: fix hardened 'mkdir' default permissions (re: 9db8cf90)
tst/run.sh: fix hardened 'mkdir' default permissions (re: 9db8cf90)

POSIX says that 'mkdir -m' takes 'a=rwx' (i.e., 777) as default.
macOS/*BSD 'mkdir' does this regardless of umask, but GNU 'mkdir'
removes permissions from the default based on the umask. Turns out
that POSIX actually specifies the GNU behaviour, but only way down
under "application usage" and not in the description of the -m
option. The GNU behaviour caused the regression test suite to die
due to permission errors.
Ref.: https://pubs.opengroup.org/onlinepubs/9699919799/utilities/mkdir.html#tag_20_79_04
      https://pubs.opengroup.org/onlinepubs/9699919799/utilities/mkdir.html#tag_20_79_16

lib/modernish/tst/run.sh:
- When hardening mkdir, just specify the permission bits in octal,
  the most widely used and well-tested form.

modernish/modernish