Add more JSON-RPC integration tests to `jrpc-echod`

[msgilligan]

Add tests for:

• help
• help <method-name> -- this will fail because help is not implemented to take a method-name parameter)
• invalid-command
• echo text extra-argument -- should return an error since there is only one argument

Note that 3 of the above tests will get failure results from the sever, but the test should pass and have condition that verify the server correctly returned failure results with the message and code.

You can use the echoMethod() test in ApplicationTest.java as a template.

Testing for stop is more complicated and is not needed to complete this issue:

• stop -- This might need to be the last test or might need to be in a separate class that starts a server just for stopping.

[msgilligan]

PR #293 approved and will be auto-merged if tests pass.

For a next step, I suggest adding the creation of the client to @BeforeEach and add an @AfterEach that calls client.close().

[liamgilligan]

See #293

[msgilligan]

For a next step, I suggest adding the creation of the client to @BeforeEach and add an @AfterEach that calls client.close().

@liamgilligan I requested this in my review of PR #300