Fix usr.bin.truncate.truncate_test.bad_truncate with ZFS /tmp.

[asomers]

Summary

Use simmutable instead of uimmutable in the bad_truncate test. The former
is supported by both ZFS and UFS, but the latter only by UFS.

[jilles]

This adds a requirement that the test be run as root and that securelevel be -1 or 0, which may not be desirable. To prevent compatibility problems, a line

TEST_METADATA.truncate_test+=  required_user="root"

should be added to usr.bin/truncate/tests/Makefile.

Alternatively, file permissions (chmod 444) instead of file flags could be used. This requires

TEST_METADATA.truncate_test+=  required_user="unprivileged"

and the expected error message should be Permission denied instead of Operation not permitted.

[asomers]

Address feedback from jilles: Use permissions instead of flags in bad_truncate

[jilles]

Looks good to me.