dm: test: Fix running of multiple test from command line

dm: test: Fix running of multiple test from command line

At present when multiple 'ut dm' commands are executed, all but the first
is run with a flat tree, even if live tree is enabled. This is because the
live tree node pointer is set to NULL and never restored.

This does not affect normal test running, which just runs all the test in
one go, but can be confusing when several individual tests are run during
the same U-Boot run.

Correct this by restoring the pointer.

Fixes: c166c47ba3 (dm: test: Add support for running tests with livetree)
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>