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/test3012

@@ -18,7 +18,7 @@ ETag: "21025-dc7-39462498"
 Accept-Ranges: bytes
 Content-Length: 6
 Connection: close
-Content-Disposition: inline; filename="MMM3012MMM"
+Content-Disposition: inline; filename="MMM%TESTNUMBERMMM"
 Content-Type: text/html
 Funny-head: yesyes
 
@@ -39,7 +39,7 @@ http
 --output-dir with -J
 </name>
 <command option="no-output,no-include">
-http://%HOSTIP:%HTTPPORT/this/is/the/3012 -OJ --output-dir %PWD/log
+http://%HOSTIP:%HTTPPORT/this/is/the/%TESTNUMBER -OJ --output-dir %PWD/log
 </command>
 </client>
 
@@ -47,13 +47,13 @@ http://%HOSTIP:%HTTPPORT/this/is/the/3012 -OJ --output-dir %PWD/log
 # Verify data after the test has been "shot"
 <verify>
 <protocol>
-GET /this/is/the/3012 HTTP/1.1
+GET /this/is/the/%TESTNUMBER HTTP/1.1
 Host: %HOSTIP:%HTTPPORT
 User-Agent: curl/%VERSION
 Accept: */*
 
 </protocol>
-<file name="log/MMM3012MMM">
+<file name="log/MMM%TESTNUMBERMMM">
 -foo-
 </file>
 </verify>