tests: use skip_test_, not skip_

tests: use skip_test_, not skip_

skip_test_ emits its diagnostic both to FD 9 (tty), and to
FD 2 (usually the log file), whereas init.sh's skip_ emits
only to FD 9.  Without that, the log is slightly less useful.
* tests/cp/fiemap-2: Use skip_test_, not skip_.
* tests/cp/fiemap-perf: Likewise.
* tests/du/bigtime: Likewise.
* tests/du/files0-from-dir: Likewise.
* tests/du/move-dir-while-traversing: Likewise.
* tests/init.sh: Likewise.
* tests/misc/sort-stale-thread-mem: Likewise.
* tests/misc/stat-nanoseconds: Likewise.
* tests/mv/i-3: Likewise.
* tests/mv/sticky-to-xpart: Likewise.
* tests/split/filter: Likewise.
Prompted by a report from Pádraig Brady.