test: Increase expect_str/actual_str buffer size

These buffers can be exceeded when checking the FIT output with an RSA
key. Increase the limit to 1K.

Co-developed-by: Claude <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>

include/test/test.h

@@ -78,8 +78,8 @@ struct unit_test_state {
 	bool force_run;
 	void *old_bloblist;
 	bool soft_fail;
-	char expect_str[512];
-	char actual_str[512];
+	char expect_str[1024];
+	char actual_str[1024];
 };
 
 /* Test flags for each test */