selftests: ktap_helpers: Make it POSIX-compliant

There are a couple uses of bash specific syntax in the script. Change
them to the equivalent POSIX syntax. This doesn't change functionality
and allows non-bash test scripts to make use of these helpers.

Reported-by: Mike Looijmans <[e-mail]>
Closes: https://lore.kernel.org/all/[e-mail]/
Fixes: 2dd0b5a8fcc4 ("selftests: ktap_helpers: Add a helper to finish the test")
Fixes: 14571ab1ad21 ("kselftest: Add new test for detecting unprobed Devicetree devices")
Signed-off-by: Nícolas F. R. A. Prado <[e-mail]>
Reviewed-by: Muhammad Usama Anjum <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/kselftest/ktap_helpers.sh

···
 	directive="$3" # optional
 
 	local directive_str=
-	[[ ! -z "$directive" ]] && directive_str="# $directive"
+	[ ! -z "$directive" ] && directive_str="# $directive"
 
 	echo $result $KTAP_TESTNO $description $directive_str
 
···
 ktap_finished() {
 	ktap_print_totals
 
-	if [ $(("$KTAP_CNT_PASS" + "$KTAP_CNT_SKIP")) -eq "$KSFT_NUM_TESTS" ]; then
+	if [ $((KTAP_CNT_PASS + KTAP_CNT_SKIP)) -eq "$KSFT_NUM_TESTS" ]; then
 		exit "$KSFT_PASS"
 	else
 		exit "$KSFT_FAIL"