Ensure use of "echo_i" where possible

In many instances 'echo "I:' construct was used where echo_i function
should have been.

bin/tests/system/zero/prereq.sh

@@ -16,6 +16,6 @@ if $PERL -e 'use Net::DNS;' 2>/dev/null
 then
     :
 else
-    echo "I:This test requires the Net::DNS library." >&2
+    echo_i "This test requires the Net::DNS library." >&2
     exit 1
 fi