ztest: add user thread support

We add macros to define test cases that should be run with the CPU
in user mode, if the CPU supports it.

ztest_test_suite() declarations are now static as they can't go on
the main thread stack; the data gets shared between multiple threads.
It's better here anyway as a large test suite could fill up the main
stack, which is by default reduced to 512 bytes.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>