add unlink / start with a clean state

... good when running "prove -v" locally and previously
the run was interrrupted by e.g. ^C

t/09_isJSON_severitylevel_valid.t

@@ -17,6 +17,7 @@ $tests = 0;
 
 
 printf "\n%s\n", "Doing severity level checks";
+unlink 'tmp.json';
 
 #1
 pass(" .. running testssl.sh against badssl.com to create a JSON report with severity level equal greater than LOW (may take 2~3 minutes)"); $tests++;