ztest: Add register functionality

Add new functionality to ztest to improve test modularity. The two
primary new entry points are:
* ztest_register_test_suite
* ztest_run_registered_test_suites

When registering a new test suite, users provide the name as well as
an optional predicate used to filter the tests for each run. Using NULL
as the predicate ensures that the test is run exactly once (after which
it is automatically filtered from future runs).

Calls to ztest_run_registered_test_suites take a state pointer as an
argument. This allows the the pragma functions to decide whether the
test should be run.

The biggest benefit of this system (other than the ability to filter
tests and maintain a larger test state) is the ability to better
modularize the test source code. Instead of all the various tests
having to coordinate and the main function having to know which tests
to run, each source file manages registering its own test
suite and handling the conditions for running the suite.

Signed-off-by: Yuval Peress <peress@chromium.org>

include/linker/common-ram.ld

@@ -140,3 +140,7 @@
 #ifdef CONFIG_USERSPACE
 	_static_kernel_objects_end = .;
 #endif
+
+#if defined(CONFIG_ZTEST)
+	ITERABLE_SECTION_RAM(ztest_suite_node, 4)
+#endif /* CONFIG_ZTEST */