test/py: test the shell if command

test/py: test the shell if command

Migrate all most tests from command_ut.c into the Python test system.
This allows the tests to be run against any U-Boot binary that supports
the if command (i.e. where hush is enabled) without requiring that
binary to be permanently bloated with the code from command_ut.

Some tests in command_ut.c can only be executed from C code, since they
test internal (more unit-level) features of various U-Boot APIs. The
migrated tests can all operate directly from the U-Boot console.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>