Test for single output is actually requesting none

[jerstlouis]

Describe the bug The Sync Raw Value One test is expecting a single output, but is actually requesting either no outputs at all, or all outputs. For this test, the ETS submits the execution request:

{
   "id":"echo",
   "inputs":{
      "stringInput":"teststring"
   },
   "outputs":{},
   "response":"raw"
}

which specifies either to return none of the outputs, or all of the outputs described in the process description. The Standard states that all outputs are returned when outputs is omitted, but does not specify what happens if an empty object is specified ( see https://github.com/opengeospatial/ogcapi-processes/issues/316 )

Normally, one would assume to be "no output", which might make sense for a process that may have a side-effect other than returning outputs. In the case of the ETS and the echo process, the intent here based on the name of the test seemed to be to request a single output.

Expected behavior The ETS should use "outputs":{ "stringInput": {} }" to reflect the likely intent to test retrieving a single output for which it specifies a corresponding input.

Additional context Somewhat related to #60.