test: Add a private buffer for tests

Add a priv[] buffer to struct unit_test_state that tests can use for
their own data. This avoids the need to allocate memory or use global
variables for test-specific state.

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

doc/develop/tests_writing.rst

@@ -486,6 +486,20 @@ ut_check_delta(last)
     Return the change in free memory since ``last`` was obtained from
     ``ut_check_free()``. A positive value means more memory has been allocated.
 
+Private buffer
+~~~~~~~~~~~~~~
+
+Each test has access to a private buffer ``uts->priv`` (256 bytes) for temporary
+data. This avoids the need to allocate memory or use global variables::
+
+   static int my_test(struct unit_test_state *uts)
+   {
+      snprintf(uts->priv, sizeof(uts->priv), "/%s", filename);
+      /* use uts->priv as a path string */
+
+      return 0;
+   }
+
 
 Writing Python tests
 --------------------

include/test/test.h

@@ -10,6 +10,7 @@
 #include <linux/bitops.h>
 
 #define UT_MAX_ARGS	8
+#define UT_PRIV_SIZE	256
 
 /**
  * struct ut_stats - Statistics about tests run
@@ -95,6 +96,7 @@ struct ut_arg {
  * @args: Parsed argument values for current test
  * @arg_count: Number of parsed arguments
  * @arg_error: Set if ut_str/int/bool() detects a type mismatch
+ * @priv: Private data for tests to use as needed
  */
 struct unit_test_state {
 	struct ut_stats cur;
@@ -124,6 +126,7 @@ struct unit_test_state {
 	struct ut_arg args[UT_MAX_ARGS];
 	int arg_count;
 	bool arg_error;
+	char priv[UT_PRIV_SIZE];
 };
 
 /* Test flags for each test */