ztest: Add macros for comparing strings

[rugeGerritsen]

These macros allows us to compare strings in a simpler way.

[bjarki-andreasen]

I can only say yes please and thank you :)

zephyr/tests/subsys/modem/modem_chat/src/main.c

Lines 524 to 528 in db57c43

	zassert_equal(strcmp(argv_buffers[0], "+CMGL: "), 0, "Incorrect argv received");
	zassert_equal(strcmp(argv_buffers[1], "1"), 0, "Incorrect argv received");
	zassert_equal(strcmp(argv_buffers[2], "1"), 0, "Incorrect argv received");
	zassert_equal(strcmp(argv_buffers[3], ""), 0, "Incorrect argv received");
	zassert_equal(strcmp(argv_buffers[4], "50"), 0, "Incorrect argv received");

[rugeGerritsen]

@bjarki-trackunit , I've created #74083 as a follow-up