ztest: Refine when to collect unit test duration

Previously, unit test duration is collected within the
TC_START and Z_TC_END_RESULT macros. With existing tests,
the TC_START macro can be invoked by both the ztest fx
and the tests themselves. And the TC_START macro definition
went lengths to avoid the interference when it is invoked
within a unit test. This commit decouple the time collection
and the TC_STRAT/Z_TC_END_RESULT macros to fix this issue.
Now only the (old) ztest framework is responsible for the
test duration measure. The test duration stats of new ztest
fx is different from this btw.

Signed-off-by: Ming Shao <ming.shao@intel.com>