plugins/t/check_dns.t - Fix Perl Warning

Resolves perl not understanding /d within "".

Apply& merge broke, this is taken from https://github.com/monitoring-plugins/monitoring-plugins/commit/c5a64eeef8340ff39fafb21878d92139b4c1fb73

plugins/t/check_dns.t

@@ -74,6 +74,7 @@ cmp_ok( $res->return_code, '==', 2, "Critical threshold passed");
 
 $res = NPTest->testCmd("./check_dns -H $hostname_valid -t 5 -w 0 -c 5");
 cmp_ok( $res->return_code, '==', 1, "Warning threshold passed");
+like( $res->output, '/\|time=[\d\.]+s;0.0*;5\.0*;0\.0*/', "Output performance data OK" );
 
 $res = NPTest->testCmd("./check_dns -H $hostname_invalid -t 1");
 cmp_ok( $res->return_code, '==', 2, "Invalid $hostname_invalid");