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

tests/test-runner/bin/zts-report.py.in

@@ -236,7 +236,7 @@ maybe = {
     'snapshot/snapshot_010_pos': ['FAIL', 7961],
     'snapused/snapused_004_pos': ['FAIL', 5513],
     'tmpfile/setup': ['SKIP', tmpfile_reason],
-    'threadsappend/threadsappend_001_pos': ['FAIL', 6136],
+    'append/threadsappend_001_pos': ['FAIL', 6136],
     'trim/setup': ['SKIP', trim_reason],
     'upgrade/upgrade_projectquota_001_pos': ['SKIP', project_id_reason],
     'user_namespace/setup': ['SKIP', user_ns_reason],