external test scripts: Replace the old dollar expressions with newer variants

scripts/common.sh

@@ -46,7 +46,7 @@ safe_kill()
     while kill -0 "$PID" 2>/dev/null && [[ $n -le 4 ]]; do
         echo "Waiting ($n) ..."
         sleep 1
-        n=$[n + 1]
+        n=$((n + 1))
     done
 
     # process still alive? then hard-kill

test/externalTests/common.sh

@@ -80,7 +80,7 @@ function download_project
     printLog "Cloning $branch of $repo..."
     git clone --depth 1 "$repo" -b "$branch" "$dir/ext"
     cd ext
-    echo "Current commit hash: `git rev-parse HEAD`"
+    echo "Current commit hash: $(git rev-parse HEAD)"
 }
 
 function force_truffle_version