unit-test: cover run_commandf() by test-cases

unit-test: cover run_commandf() by test-cases

As run_commandf() is variadic version of run_command() and just a wrapper,
hence apply similar run_command's test-cases.

Let's avoid warning about empty string passing:
warning: zero-length gnu_printf format string [-Wformat-zero-length]
   assert(run_commandf("") == 0);

Signed-off-by: Evgeny Bachinin <EABachinin@sberdevices.ru>
Reviewed-by: Simon Glass <sjg@chromium.org>