test/boot: Make BOOTSTD tests depend on UT_BOOTSTD

While we have a symbol for controlling if we will be testing BOOTSTD or
not, and it depends on SANDBOX, we do not control the building of
test/boot/ content correctly. Guard the current bootstd tests with
a check for UT_BOOTSTD and leave the measurement test available.

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