t_create.sh: use `ATF_TESTS_SH_SED` & remove local mods

This particular change replaces all local modifications to the test
script like so:
- Use `ATF_TESTS_SH_SED_test` with a sed(1) statement in t

t_create.sh: use `ATF_TESTS_SH_SED` & remove local mods

This particular change replaces all local modifications to the test
script like so:
- Use `ATF_TESTS_SH_SED_test` with a sed(1) statement in the Makefile,
instead of the equivalent local modifications.
- Remove the need for expecting the output of newfs_msdos to be empty.
There isn't much to gain from deviating from the upstream NetBSD test--it's
just another local modification that would need to be carried forward. If
it's worth testing this FreeBSD-specific behavior, it should be in a
FreeBSD-specific test.

This makes moving new modifications to the script easier moving forward.

MFC after: 1 week

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