subsys/testsuite: make tc_util overridable

A Kconfig boolean is added to allow users to provide their own
output strings when running tests via ztest.
This allows changing e.g. the PASS/FAIL/SKIPPED strings,
add counters, change separators, and similar.

A test using the feature and relevant documentation is added.

Signed-off-by: Torstein Grindvik <torstein.grindvik@nordicsemi.no>