tests/ztest: rename assert macros to be zephyr specific

ztest has a number of assert style macros and used a baseline assert()
that varies from the system definition of assert() so lets rename
everything as zassert to be clear.

Change-Id: I7f176b3bae94d1045054d665be8b5bda947e5bb0
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>