Get rid of bashisms in string comparisons

The double equal sign ('==') is a Bash-specific string comparison
operator.  Ensure the single equal sign ('=') is used in all POSIX shell
scripts in the system test suite in order to retain their portability.

(cherry picked from commit 481dfb9671)

bin/tests/system/rpz/tests.sh

@@ -209,7 +209,7 @@ restart () {
 	fi
     fi
     rm -f ns$1/*.jnl
-    if [ "$2" == "rebuild-bl-rpz" ]; then
+    if [ "$2" = "rebuild-bl-rpz" ]; then
         if test -f ns$1/base.db; then
 	    for NM in ns$1/bl*.db; do
 	        cp -f ns$1/base.db $NM