After #1545 disabled some tests for reply schema validation, we now have
another issue that ECHO is not covered.
```
WARNING! The following commands were not hit at all:
echo
ERROR! at least one command was not hit by the tests
```
This patch adds a test case for ECHO in the unit/other test suite. I
haven't checked if there are more commands that aren't covered.
Signed-off-by: Viktor Söderqvist <viktor.soderqvist@est.tech>
dc9ca1b98d