add a test for stderr output to the cmd_exec tests

[timwr]

This change adds a test to ensure the stderr output can be seen when running cmd_exec.
This was broken on Java and fixed with rapid7/metasploit-payloads#393
Additionally this was broken on the cmd/windows/reverse_powershell payload before #12985 fixed it.

Verification

List the steps needed to make sure this thing works

• Get a java meterpreter session (on Windows or Linux) msfconsole -qx "use exploit/multi/handler; set payload java/meterpreter/reverse_tcp; set lhost $LHOST; set lport 4444; set ExitOnSession false; run -j"
• Run the following:

loadpath test/modules
use post/test/cmd_exec 
set SESSION -1
run

• Verify the tests all pass

[busterb]

Release Notes

This adds tests to verify that payloads, when used with the cmd_exec API, return the output of the stderr process stream in their results.