This is a variant of `g_test_trap_subprocess()` which allows the
environment for the child process to be specified. This is quite useful
when you want to test code which reads environment variables, as it’s
not safe to set those after the start of `main()`.

This will be useful within and outwith GLib for testing such code.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #1618