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/test508

@@ -27,14 +27,14 @@ http
 </server>
 # tool is what to use instead of 'curl'
 <tool>
-lib508
+lib%TESTNUMBER
 </tool>
 
  <name>
 send HTTP POST using read callback
  </name>
  <command>
-http://%HOSTIP:%HTTPPORT/508
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER
 </command>
 </client>
 
@@ -46,7 +46,7 @@ http://%HOSTIP:%HTTPPORT/508
 s/^(this is what we post to the silly web server)\r\n/$1\n/ if($has_charconv)
 </strippart>
 <protocol>
-POST /508 HTTP/1.1
+POST /%TESTNUMBER HTTP/1.1
 Host: %HOSTIP:%HTTPPORT
 Accept: */*
 Content-Length: 45