perf tests: Address signal case issues detected via shellcheck

Running shellcheck -S on test_arm_spe_fork.sh throws below warnings:

   In tests/shell/test_arm_spe_fork.sh line 25:
   trap cleanup_files 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.

Fixed this issue by using uppercase for "EXIT", "TERM" and
"INIT" signals to avoid using lower/mixed case for signal
names as input.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Disha Goel <disgoel@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kajol Jain <kjain@linux.ibm.com>
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/20230709182800.53002-7-atrajeev@linux.vnet.ibm.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>