The standard test in the suite is the "curl test". They all invoke a curl command line and verifies that everything it sends, gets back and returns are exactly as expected. Any mismatch and the test is considered a fail and the script shows details about the error.
What the test features in the <client><command> section is what is used in
the command line, verbatim.
The tests/log/commands.log is handy to look at after a run, as it contains
the full command line that was run in the test.
If you want to make a test that does not invoke the curl command line tool, then you should consider the libcurl tests or unit tests instead.