tests/shell: Fix shellcheck warnings for SC2153 in multiple scripts

tests/shell: Fix shellcheck warnings for SC2153 in multiple scripts

Running shellcheck on some of the shell scripts, throws
below warning on shellcheck v0.6. Example:

   In tests/shell/coresight/asm_pure_loop.sh line 14:
   DATA="$DATD/perf-$TEST-$DATV.data"
          ^---^ SC2153: Possible misspelling: DATD may not be assigned, but DATA is.

Here, DATD is exported from "lib/coresight.sh" and this
warning can be ignored. Use "shellcheck disable=" to ignore
this check.

Signed-off-by: Athira Rajeev <atrajeev@linux.vnet.ibm.com>
Tested-by: Ian Rogers <irogers@google.com>
Reviewed-by: Kajol Jain <kjain@linux.ibm.com>
Cc: maddy@linux.ibm.com
Cc: disgoel@linux.vnet.ibm.com
Cc: linuxppc-dev@lists.ozlabs.org
Link: https://lore.kernel.org/r/20230907171540.36736-4-atrajeev@linux.vnet.ibm.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>