string: Convert helpers selftest to KUnit

Convert test-string_helpers.c to KUnit so it can be easily run with
everything else.

Failure reporting doesn't need to be open-coded in most places, for
example, forcing a failure in the expected output for upper/lower
testing looks like this:

[12:18:43] # test_upper_lower: EXPECTATION FAILED at lib/string_helpers_kunit.c:579
[12:18:43] Expected dst == strings_upper[i].out, but
[12:18:43]     dst == "ABCDEFGH1234567890TEST"
[12:18:43]     strings_upper[i].out == "ABCDEFGH1234567890TeST"
[12:18:43] [FAILED] test_upper_lower

Currently passes without problems:

$ ./tools/testing/kunit/kunit.py run string_helpers
...
[12:23:55] Starting KUnit Kernel (1/1)...
[12:23:55] ============================================================
[12:23:55] =============== string_helpers (3 subtests) ================
[12:23:55] [PASSED] test_get_size
[12:23:55] [PASSED] test_upper_lower
[12:23:55] [PASSED] test_unescape
[12:23:55] ================= [PASSED] string_helpers ==================
[12:23:55] ============================================================
[12:23:55] Testing complete. Ran 3 tests: passed: 3
[12:23:55] Elapsed time: 6.709s total, 0.001s configuring, 6.591s building, 0.066s running

Link: https://lore.kernel.org/r/20240301202732.2688342-2-keescook@chromium.orgSigned-off-by: Kees Cook <keescook@chromium.org>

MAINTAINERS

@@ -8978,7 +8978,7 @@ F:	include/linux/string_helpers.h
 F:	lib/string.c
 F:	lib/string_kunit.c
 F:	lib/string_helpers.c
-F:	lib/test-string_helpers.c
+F:	lib/string_helpers_kunit.c
 F:	scripts/coccinelle/api/string_choices.cocci
 
 GENERIC UIO DRIVER FOR PCI DEVICES

lib/Kconfig.debug

@@ -2357,8 +2357,10 @@ config STRING_KUNIT_TEST
 	depends on KUNIT
 	default KUNIT_ALL_TESTS
 
-config TEST_STRING_HELPERS
-	tristate "Test functions located in the string_helpers module at runtime"
+config STRING_HELPERS_KUNIT_TEST
+	tristate "KUnit test string helpers at runtime" if !KUNIT_ALL_TESTS
+	depends on KUNIT
+	default KUNIT_ALL_TESTS
 
 config TEST_KSTRTOX
 	tristate "Test kstrto*() family of functions at runtime"

lib/Makefile

@@ -51,7 +51,7 @@ obj-y += bcd.o sort.o parser.o debug_locks.o random32.o \
 	 generic-radix-tree.o bitmap-str.o
 obj-$(CONFIG_STRING_KUNIT_TEST) += string_kunit.o
 obj-y += string_helpers.o
-obj-$(CONFIG_TEST_STRING_HELPERS) += test-string_helpers.o
+obj-$(CONFIG_STRING_HELPERS_KUNIT_TEST) += string_helpers_kunit.o
 obj-y += hexdump.o
 obj-$(CONFIG_TEST_HEXDUMP) += test_hexdump.o
 obj-y += kstrtox.o