Rename stage-tmpfile to o_tmpfile

We had a one-off test that was overly specific to staging from tmpfile.
This renames it to a more generic test where we can add more tests of
o_tmpfile in general.

Signed-off-by: Zach Brown <zab@versity.com>

tests/sequence

@@ -27,7 +27,7 @@ createmany-large-names.sh
 createmany-rename-large-dir.sh
 stage-release-race-alloc.sh
 stage-multi-part.sh
-stage-tmpfile.sh
+o_tmpfile.sh
 basic-posix-consistency.sh
 dirent-consistency.sh
 mkdir-rename-rmdir.sh