doc: pytest: Document the test_net test

Add this test to the documentation. While the diff appears large at
first, the only changes within the test are to move the imports to
follow the pydoc comment and then to code-block and indent the example
configuration.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/develop/pytest/index.rst b/doc/develop/pytest/index.rst
index 3608388..e2131a2 100644
--- a/doc/develop/pytest/index.rst
+++ b/doc/develop/pytest/index.rst
@@ -18,4 +18,5 @@
    :maxdepth: 1
 
    test_000_version
+   test_net
    test_net_boot