test/telemetry: add unit tests for data to JSON

test/telemetry: add unit tests for data to JSON

This patch adds tests for verifying telemetry data structures are
converted to JSON as expected. Both flat and recursive data structures
are tested, for all possible value types.

The app connects to the telemetry socket as a client, and registers one
command with a corresponding callback function. Each time the callback
function is called, it copies a global data variable to the data pointer
passed in by telemetry.
When a test case is run, the test case function builds up the global
data variable with the relevant data types, and the expected json string
output which should be generated from that. The 'test_output()' function
is used to trigger the callback and ensure the actual output matches
that expected.

Signed-off-by: Louise Kilheeney <louise.kilheeney@intel.com>
Signed-off-by: Ciara Power <ciara.power@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>