rez-env output test is not testing on specified shell

[davidlatwe]

In test case test_shells.TestShells.test_rez_env_output, it uses decorator per_available_shell, which iterates all available shells and use config.override to set testing target.

https://github.com/nerdvegas/rez/blob/8f965f6ed7ac438c1ea331940e6ad14f766564fa/src/rez/tests/util.py#L218-L221

However, when testing rez-env output, subprocess is called which actually won't take the config override and always testing on system default shell.

https://github.com/nerdvegas/rez/blob/8f965f6ed7ac438c1ea331940e6ad14f766564fa/src/rez/tests/test_shells.py#L183-L190

The var arg in above code should changed into

   ...
     def _test(txt): 
         ...
         args = [binpath, "--shell", config.default_shell, "--", "echo", txt]

I found this issue when I was tweaking code in CMD shell class and somehow the test result of PowerShell also gets affected and made me confused.