GSubprocessLauncher: don't get empty environment

Use g_get_environ() to get the environment variables with their values
instead of g_listenv() which only lists off the keys.

https://bugzilla.gnome.org/show_bug.cgi?id=725651
gsubprocess: test environment a bit more

Add a test for GSubprocess to test setting, unsetting and inheritance of
environment variables.  Use communicate() to give it a bit more of a
workout as well.

https://bugzilla.gnome.org/show_bug.cgi?id=725651