Fix "coverage -h" command

$SCRIPT_NAME is not defined so causes an error. Use $(basename "$0") instead.

maint/coverage

@@ -3,7 +3,7 @@
 function usage()
 {
     cat <<EOF
-${SCRIPT_NAME}: Generate coverage using grcov.
+$(basename "$0"): Generate coverage using grcov.
 
 Usage:
   coverage [opts] [suites...] : Run the provided test suites.