vboot/ui: Refine mock functions in UI tests

Rearrange and split some action tests.

Rewrite mock_ui_context related functions, and allow change_screen to
change to real screens in vb2_ui_action_tests.c.

TEST=make clean && make runtests
TEST=make clean && DETACHABLE=1; make runtests

Signed-off-by: Hsuan Ting Chen <roccochen@chromium.org>
Change-Id: I1692c9fcdf19073f8a9810dfab769df4c87224a9
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/vboot_reference/+/2209780
Reviewed-by: Yu-Ping Wu <yupingso@chromium.org>
3 files changed