tests: use %TESTNUMBER instead of fixed number

This makes the tests easier to copy and relocate to other test numbers
without having to update content.

Closes #6738

tests/data/test306

@@ -46,7 +46,7 @@ https
 HTTPS GET, receive no headers only data!
  </name>
  <command>
--k https://%HOSTIP:%HTTPSPORT/306 --http0.9
+-k https://%HOSTIP:%HTTPSPORT/%TESTNUMBER --http0.9
 </command>
 </client>
 
@@ -54,7 +54,7 @@ HTTPS GET, receive no headers only data!
 # Verify data after the test has been "shot"
 <verify>
 <protocol>
-GET /306 HTTP/1.1
+GET /%TESTNUMBER HTTP/1.1
 Host: %HOSTIP:%HTTPSPORT
 User-Agent: curl/%VERSION
 Accept: */*