Test some more GSubProcess api

This covers g_subprocess_set_environ, g_subprocess_setenv,
g_subprocess_getenv, and g_subprocess_set_flags.
Fix g_subprocess_launcher_spawn

This function turns a varargs argument list into a string array,
but forgets to NULL-terminate it. This function was not covered
by unit tests...so it was broken.
Test g_subprocess_launcher_spawn

The spawnv variant of this function already had a test, but
spawn was not tested.