selftests/bpf: Add ASSERT_STRNEQ() variant for test_progs

It will support strncmp()-style string comparisons.
Suggested-by: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/1626362126-27775-3-git-send-email-alan.maguire@oracle.com

tools/testing/selftests/bpf/test_progs.h

@@ -221,6 +221,18 @@ extern int test__join_cgroup(const char *path);
 	___ok;								\
 })
 
+#define ASSERT_STRNEQ(actual, expected, len, name) ({			\
+	static int duration = 0;					\
+	const char *___act = actual;					\
+	const char *___exp = expected;					\
+	int ___len = len;						\
+	bool ___ok = strncmp(___act, ___exp, ___len) == 0;		\
+	CHECK(!___ok, (name),						\
+	      "unexpected %s: actual '%.*s' != expected '%.*s'\n",	\
+	      (name), ___len, ___act, ___len, ___exp);			\
+	___ok;								\
+})
+
 #define ASSERT_OK(res, name) ({						\
 	static int duration = 0;					\
 	long long ___res = (res);					\