perf tests record_offcpu: Fix shellcheck warnings about word splitting/quoting and signal names case

Running shellcheck on record_offcpu.sh throws below warning:

In tests/shell/record_offcpu.sh line 13:
  trap - exit term int
         ^--^ SC3049 (warning): In POSIX sh, using lower/mixed case for signal names is undefined.
              ^--^ SC3049 (warning): In POSIX sh, using lower/mixed case for signal names is undefined.
                   ^-^ SC3049 (warning): In POSIX sh, using lower/mixed case for signal names is undefined.

In tests/shell/record_offcpu.sh line 20:
trap trap_cleanup exit term int
                  ^--^ SC3049 (warning): In POSIX sh, using lower/mixed case for signal names is undefined.
                       ^--^ SC3049 (warning): In POSIX sh, using lower/mixed case for signal names is undefined.
                            ^-^ SC3049 (warning): In POSIX sh, using lower/mixed case for signal names is undefined.

In tests/shell/record_offcpu.sh line 25:
  if [ `id -u` != 0 ]
       ^-----^ SC2046 (warning): Quote this to prevent word splitting.

Fixed the warnings by:
- Capitalize signals(INT, TERM, EXIT) to avoid mixed/lower case naming of
signals.
- Adding quotes to avoid word splitting.

Result from shellcheck after patch changes:
     $ shellcheck -S warning record_offcpu.sh
     $
Signed-off-by: Kajol Jain <kjain@linux.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230709182800.53002-4-atrajeev@linux.vnet.ibm.comSigned-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>