doc: pytest: Document the test_efi_loader test

doc: pytest: Document the test_efi_loader test

Add this test to the documentation. We need to add a code-block
annotation to the example and indent it correctly. We also need to
document the do_test_efi_helloworld_net function and that in turn means
changing the documentation to test_efi_helloworld_net_http and
test_efi_helloworld_net_tftp to reflect what is and isn't done in those
functions themselves now.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>