Adding ZTS test for O_APPEND

Commit 63b18e4 fixed an issue in zpl_aio_write() to make sure that
kiocb->ki_pos was updated correctly when opening a file with O_APPEND.
Adding a test to verify O_APPEND functionality with lseek can make
sure that all other distros/kernel versions also have the correct
behavior.

Also moved the threadappends_001_pos test into this append test
directory in functional ZTS directory. This way the two append tests
are together for organization purposes.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Brian Atkinson <batkinson@lanl.gov>
Closes #13424