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/test397

@@ -174,7 +174,7 @@ http
 HTTP GET zstd compressed content of size more than CURL_MAX_WRITE_SIZE
  </name>
  <command>
-http://%HOSTIP:%HTTPPORT/397 --compressed
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER --compressed
 </command>
 </client>
 
@@ -185,7 +185,7 @@ http://%HOSTIP:%HTTPPORT/397 --compressed
 s/^Accept-Encoding: .*/Accept-Encoding: xxx/
 </strippart>
 <protocol>
-GET /397 HTTP/1.1
+GET /%TESTNUMBER HTTP/1.1
 Host: %HOSTIP:%HTTPPORT
 User-Agent: curl/%VERSION
 Accept: */*